FreeBSD/Linux Kernel Cross Reference
sys/block/ll_rw_blk.c

     /*
      * Copyright (C) 1991, 1992 Linus Torvalds
      * Copyright (C) 1994,      Karl Keyte: Added support for disk statistics
      * Elevator latency, (C) 2000  Andrea Arcangeli <andrea@suse.de> SuSE
      * Queue request tables / lock, selectable elevator, Jens Axboe <axboe@suse.de>
      * kernel-doc documentation started by NeilBrown <neilb@cse.unsw.edu.au> -  July2000
      * bio rewrite, highmem i/o, etc, Jens Axboe <axboe@suse.de> - may 2001
      */
     
    /*
     * This handles all read/write requests to block devices
     */
    #include <linux/kernel.h>
    #include <linux/module.h>
    #include <linux/backing-dev.h>
    #include <linux/bio.h>
    #include <linux/blkdev.h>
    #include <linux/highmem.h>
    #include <linux/mm.h>
    #include <linux/kernel_stat.h>
    #include <linux/string.h>
    #include <linux/init.h>
    #include <linux/bootmem.h>      /* for max_pfn/max_low_pfn */
    #include <linux/completion.h>
    #include <linux/slab.h>
    #include <linux/swap.h>
    #include <linux/writeback.h>
    #include <linux/task_io_accounting_ops.h>
    #include <linux/interrupt.h>
    #include <linux/cpu.h>
    #include <linux/blktrace_api.h>
    #include <linux/fault-inject.h>
    #include <linux/scatterlist.h>
    
    /*
     * for max sense size
     */
    #include <scsi/scsi_cmnd.h>
    
    static void blk_unplug_work(struct work_struct *work);
    static void blk_unplug_timeout(unsigned long data);
    static void drive_stat_acct(struct request *rq, int new_io);
    static void init_request_from_bio(struct request *req, struct bio *bio);
    static int __make_request(struct request_queue *q, struct bio *bio);
    static struct io_context *current_io_context(gfp_t gfp_flags, int node);
    static void blk_recalc_rq_segments(struct request *rq);
    static void blk_rq_bio_prep(struct request_queue *q, struct request *rq,
                                struct bio *bio);
    
    /*
     * For the allocated request tables
     */
    static struct kmem_cache *request_cachep;
    
    /*
     * For queue allocation
     */
    static struct kmem_cache *requestq_cachep;
    
    /*
     * For io context allocations
     */
    static struct kmem_cache *iocontext_cachep;
    
    /*
     * Controlling structure to kblockd
     */
    static struct workqueue_struct *kblockd_workqueue;
    
    unsigned long blk_max_low_pfn, blk_max_pfn;
    
    EXPORT_SYMBOL(blk_max_low_pfn);
    EXPORT_SYMBOL(blk_max_pfn);
    
    static DEFINE_PER_CPU(struct list_head, blk_cpu_done);
    
    /* Amount of time in which a process may batch requests */
    #define BLK_BATCH_TIME  (HZ/50UL)
    
    /* Number of requests a "batching" process may submit */
    #define BLK_BATCH_REQ   32
    
    /*
     * Return the threshold (number of used requests) at which the queue is
     * considered to be congested.  It include a little hysteresis to keep the
     * context switch rate down.
     */
    static inline int queue_congestion_on_threshold(struct request_queue *q)
    {
            return q->nr_congestion_on;
    }
    
    /*
     * The threshold at which a queue is considered to be uncongested
     */
    static inline int queue_congestion_off_threshold(struct request_queue *q)
    {
            return q->nr_congestion_off;
    }
   
   static void blk_queue_congestion_threshold(struct request_queue *q)
   {
           int nr;
   
           nr = q->nr_requests - (q->nr_requests / 8) + 1;
           if (nr > q->nr_requests)
                   nr = q->nr_requests;
           q->nr_congestion_on = nr;
   
           nr = q->nr_requests - (q->nr_requests / 8) - (q->nr_requests / 16) - 1;
           if (nr < 1)
                   nr = 1;
           q->nr_congestion_off = nr;
   }
   
   /**
    * blk_get_backing_dev_info - get the address of a queue's backing_dev_info
    * @bdev:       device
    *
    * Locates the passed device's request queue and returns the address of its
    * backing_dev_info
    *
    * Will return NULL if the request queue cannot be located.
    */
   struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev)
   {
           struct backing_dev_info *ret = NULL;
           struct request_queue *q = bdev_get_queue(bdev);
   
           if (q)
                   ret = &q->backing_dev_info;
           return ret;
   }
   EXPORT_SYMBOL(blk_get_backing_dev_info);
   
   /**
    * blk_queue_prep_rq - set a prepare_request function for queue
    * @q:          queue
    * @pfn:        prepare_request function
    *
    * It's possible for a queue to register a prepare_request callback which
    * is invoked before the request is handed to the request_fn. The goal of
    * the function is to prepare a request for I/O, it can be used to build a
    * cdb from the request data for instance.
    *
    */
   void blk_queue_prep_rq(struct request_queue *q, prep_rq_fn *pfn)
   {
           q->prep_rq_fn = pfn;
   }
   
   EXPORT_SYMBOL(blk_queue_prep_rq);
   
   /**
    * blk_queue_merge_bvec - set a merge_bvec function for queue
    * @q:          queue
    * @mbfn:       merge_bvec_fn
    *
    * Usually queues have static limitations on the max sectors or segments that
    * we can put in a request. Stacking drivers may have some settings that
    * are dynamic, and thus we have to query the queue whether it is ok to
    * add a new bio_vec to a bio at a given offset or not. If the block device
    * has such limitations, it needs to register a merge_bvec_fn to control
    * the size of bio's sent to it. Note that a block device *must* allow a
    * single page to be added to an empty bio. The block device driver may want
    * to use the bio_split() function to deal with these bio's. By default
    * no merge_bvec_fn is defined for a queue, and only the fixed limits are
    * honored.
    */
   void blk_queue_merge_bvec(struct request_queue *q, merge_bvec_fn *mbfn)
   {
           q->merge_bvec_fn = mbfn;
   }
   
   EXPORT_SYMBOL(blk_queue_merge_bvec);
   
   void blk_queue_softirq_done(struct request_queue *q, softirq_done_fn *fn)
   {
           q->softirq_done_fn = fn;
   }
   
   EXPORT_SYMBOL(blk_queue_softirq_done);
   
   /**
    * blk_queue_make_request - define an alternate make_request function for a device
    * @q:  the request queue for the device to be affected
    * @mfn: the alternate make_request function
    *
    * Description:
    *    The normal way for &struct bios to be passed to a device
    *    driver is for them to be collected into requests on a request
    *    queue, and then to allow the device driver to select requests
    *    off that queue when it is ready.  This works well for many block
    *    devices. However some block devices (typically virtual devices
    *    such as md or lvm) do not benefit from the processing on the
    *    request queue, and are served best by having the requests passed
    *    directly to them.  This can be achieved by providing a function
    *    to blk_queue_make_request().
    *
    * Caveat:
    *    The driver that does this *must* be able to deal appropriately
    *    with buffers in "highmemory". This can be accomplished by either calling
    *    __bio_kmap_atomic() to get a temporary kernel mapping, or by calling
    *    blk_queue_bounce() to create a buffer in normal memory.
    **/
   void blk_queue_make_request(struct request_queue * q, make_request_fn * mfn)
   {
           /*
            * set defaults
            */
           q->nr_requests = BLKDEV_MAX_RQ;
           blk_queue_max_phys_segments(q, MAX_PHYS_SEGMENTS);
           blk_queue_max_hw_segments(q, MAX_HW_SEGMENTS);
           q->make_request_fn = mfn;
           q->backing_dev_info.ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
           q->backing_dev_info.state = 0;
           q->backing_dev_info.capabilities = BDI_CAP_MAP_COPY;
           blk_queue_max_sectors(q, SAFE_MAX_SECTORS);
           blk_queue_hardsect_size(q, 512);
           blk_queue_dma_alignment(q, 511);
           blk_queue_congestion_threshold(q);
           q->nr_batching = BLK_BATCH_REQ;
   
           q->unplug_thresh = 4;           /* hmm */
           q->unplug_delay = (3 * HZ) / 1000;      /* 3 milliseconds */
           if (q->unplug_delay == 0)
                   q->unplug_delay = 1;
   
           INIT_WORK(&q->unplug_work, blk_unplug_work);
   
           q->unplug_timer.function = blk_unplug_timeout;
           q->unplug_timer.data = (unsigned long)q;
   
           /*
            * by default assume old behaviour and bounce for any highmem page
            */
           blk_queue_bounce_limit(q, BLK_BOUNCE_HIGH);
   }
   
   EXPORT_SYMBOL(blk_queue_make_request);
   
   static void rq_init(struct request_queue *q, struct request *rq)
   {
           INIT_LIST_HEAD(&rq->queuelist);
           INIT_LIST_HEAD(&rq->donelist);
   
           rq->errors = 0;
           rq->bio = rq->biotail = NULL;
           INIT_HLIST_NODE(&rq->hash);
           RB_CLEAR_NODE(&rq->rb_node);
           rq->ioprio = 0;
           rq->buffer = NULL;
           rq->ref_count = 1;
           rq->q = q;
           rq->special = NULL;
           rq->data_len = 0;
           rq->data = NULL;
           rq->nr_phys_segments = 0;
           rq->sense = NULL;
           rq->end_io = NULL;
           rq->end_io_data = NULL;
           rq->completion_data = NULL;
           rq->next_rq = NULL;
   }
   
   /**
    * blk_queue_ordered - does this queue support ordered writes
    * @q:        the request queue
    * @ordered:  one of QUEUE_ORDERED_*
    * @prepare_flush_fn: rq setup helper for cache flush ordered writes
    *
    * Description:
    *   For journalled file systems, doing ordered writes on a commit
    *   block instead of explicitly doing wait_on_buffer (which is bad
    *   for performance) can be a big win. Block drivers supporting this
    *   feature should call this function and indicate so.
    *
    **/
   int blk_queue_ordered(struct request_queue *q, unsigned ordered,
                         prepare_flush_fn *prepare_flush_fn)
   {
           if (ordered & (QUEUE_ORDERED_PREFLUSH | QUEUE_ORDERED_POSTFLUSH) &&
               prepare_flush_fn == NULL) {
                   printk(KERN_ERR "blk_queue_ordered: prepare_flush_fn required\n");
                   return -EINVAL;
           }
   
           if (ordered != QUEUE_ORDERED_NONE &&
               ordered != QUEUE_ORDERED_DRAIN &&
               ordered != QUEUE_ORDERED_DRAIN_FLUSH &&
               ordered != QUEUE_ORDERED_DRAIN_FUA &&
               ordered != QUEUE_ORDERED_TAG &&
               ordered != QUEUE_ORDERED_TAG_FLUSH &&
               ordered != QUEUE_ORDERED_TAG_FUA) {
                   printk(KERN_ERR "blk_queue_ordered: bad value %d\n", ordered);
                   return -EINVAL;
           }
   
           q->ordered = ordered;
           q->next_ordered = ordered;
           q->prepare_flush_fn = prepare_flush_fn;
   
           return 0;
   }
   
   EXPORT_SYMBOL(blk_queue_ordered);
   
   /*
    * Cache flushing for ordered writes handling
    */
   inline unsigned blk_ordered_cur_seq(struct request_queue *q)
   {
           if (!q->ordseq)
                   return 0;
           return 1 << ffz(q->ordseq);
   }
   
   unsigned blk_ordered_req_seq(struct request *rq)
   {
           struct request_queue *q = rq->q;
   
           BUG_ON(q->ordseq == 0);
   
           if (rq == &q->pre_flush_rq)
                   return QUEUE_ORDSEQ_PREFLUSH;
           if (rq == &q->bar_rq)
                   return QUEUE_ORDSEQ_BAR;
           if (rq == &q->post_flush_rq)
                   return QUEUE_ORDSEQ_POSTFLUSH;
   
           /*
            * !fs requests don't need to follow barrier ordering.  Always
            * put them at the front.  This fixes the following deadlock.
            *
            * http://thread.gmane.org/gmane.linux.kernel/537473
            */
           if (!blk_fs_request(rq))
                   return QUEUE_ORDSEQ_DRAIN;
   
           if ((rq->cmd_flags & REQ_ORDERED_COLOR) ==
               (q->orig_bar_rq->cmd_flags & REQ_ORDERED_COLOR))
                   return QUEUE_ORDSEQ_DRAIN;
           else
                   return QUEUE_ORDSEQ_DONE;
   }
   
   void blk_ordered_complete_seq(struct request_queue *q, unsigned seq, int error)
   {
           struct request *rq;
           int uptodate;
   
           if (error && !q->orderr)
                   q->orderr = error;
   
           BUG_ON(q->ordseq & seq);
           q->ordseq |= seq;
   
           if (blk_ordered_cur_seq(q) != QUEUE_ORDSEQ_DONE)
                   return;
   
           /*
            * Okay, sequence complete.
            */
           uptodate = 1;
           if (q->orderr)
                   uptodate = q->orderr;
   
           q->ordseq = 0;
           rq = q->orig_bar_rq;
   
           end_that_request_first(rq, uptodate, rq->hard_nr_sectors);
           end_that_request_last(rq, uptodate);
   }
   
   static void pre_flush_end_io(struct request *rq, int error)
   {
           elv_completed_request(rq->q, rq);
           blk_ordered_complete_seq(rq->q, QUEUE_ORDSEQ_PREFLUSH, error);
   }
   
   static void bar_end_io(struct request *rq, int error)
   {
           elv_completed_request(rq->q, rq);
           blk_ordered_complete_seq(rq->q, QUEUE_ORDSEQ_BAR, error);
   }
   
   static void post_flush_end_io(struct request *rq, int error)
   {
           elv_completed_request(rq->q, rq);
           blk_ordered_complete_seq(rq->q, QUEUE_ORDSEQ_POSTFLUSH, error);
   }
   
   static void queue_flush(struct request_queue *q, unsigned which)
   {
           struct request *rq;
           rq_end_io_fn *end_io;
   
           if (which == QUEUE_ORDERED_PREFLUSH) {
                   rq = &q->pre_flush_rq;
                   end_io = pre_flush_end_io;
           } else {
                   rq = &q->post_flush_rq;
                   end_io = post_flush_end_io;
           }
   
           rq->cmd_flags = REQ_HARDBARRIER;
           rq_init(q, rq);
           rq->elevator_private = NULL;
           rq->elevator_private2 = NULL;
           rq->rq_disk = q->bar_rq.rq_disk;
           rq->end_io = end_io;
           q->prepare_flush_fn(q, rq);
   
           elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
   }
   
   static inline struct request *start_ordered(struct request_queue *q,
                                               struct request *rq)
   {
           q->orderr = 0;
           q->ordered = q->next_ordered;
           q->ordseq |= QUEUE_ORDSEQ_STARTED;
   
           /*
            * Prep proxy barrier request.
            */
           blkdev_dequeue_request(rq);
           q->orig_bar_rq = rq;
           rq = &q->bar_rq;
           rq->cmd_flags = 0;
           rq_init(q, rq);
           if (bio_data_dir(q->orig_bar_rq->bio) == WRITE)
                   rq->cmd_flags |= REQ_RW;
           if (q->ordered & QUEUE_ORDERED_FUA)
                   rq->cmd_flags |= REQ_FUA;
           rq->elevator_private = NULL;
           rq->elevator_private2 = NULL;
           init_request_from_bio(rq, q->orig_bar_rq->bio);
           rq->end_io = bar_end_io;
   
           /*
            * Queue ordered sequence.  As we stack them at the head, we
            * need to queue in reverse order.  Note that we rely on that
            * no fs request uses ELEVATOR_INSERT_FRONT and thus no fs
            * request gets inbetween ordered sequence. If this request is
            * an empty barrier, we don't need to do a postflush ever since
            * there will be no data written between the pre and post flush.
            * Hence a single flush will suffice.
            */
           if ((q->ordered & QUEUE_ORDERED_POSTFLUSH) && !blk_empty_barrier(rq))
                   queue_flush(q, QUEUE_ORDERED_POSTFLUSH);
           else
                   q->ordseq |= QUEUE_ORDSEQ_POSTFLUSH;
   
           elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
   
           if (q->ordered & QUEUE_ORDERED_PREFLUSH) {
                   queue_flush(q, QUEUE_ORDERED_PREFLUSH);
                   rq = &q->pre_flush_rq;
           } else
                   q->ordseq |= QUEUE_ORDSEQ_PREFLUSH;
   
           if ((q->ordered & QUEUE_ORDERED_TAG) || q->in_flight == 0)
                   q->ordseq |= QUEUE_ORDSEQ_DRAIN;
           else
                   rq = NULL;
   
           return rq;
   }
   
   int blk_do_ordered(struct request_queue *q, struct request **rqp)
   {
           struct request *rq = *rqp;
           const int is_barrier = blk_fs_request(rq) && blk_barrier_rq(rq);
   
           if (!q->ordseq) {
                   if (!is_barrier)
                           return 1;
   
                   if (q->next_ordered != QUEUE_ORDERED_NONE) {
                           *rqp = start_ordered(q, rq);
                           return 1;
                   } else {
                           /*
                            * This can happen when the queue switches to
                            * ORDERED_NONE while this request is on it.
                            */
                           blkdev_dequeue_request(rq);
                           end_that_request_first(rq, -EOPNOTSUPP,
                                                  rq->hard_nr_sectors);
                           end_that_request_last(rq, -EOPNOTSUPP);
                           *rqp = NULL;
                           return 0;
                   }
           }
   
           /*
            * Ordered sequence in progress
            */
   
           /* Special requests are not subject to ordering rules. */
           if (!blk_fs_request(rq) &&
               rq != &q->pre_flush_rq && rq != &q->post_flush_rq)
                   return 1;
   
           if (q->ordered & QUEUE_ORDERED_TAG) {
                   /* Ordered by tag.  Blocking the next barrier is enough. */
                   if (is_barrier && rq != &q->bar_rq)
                           *rqp = NULL;
           } else {
                   /* Ordered by draining.  Wait for turn. */
                   WARN_ON(blk_ordered_req_seq(rq) < blk_ordered_cur_seq(q));
                   if (blk_ordered_req_seq(rq) > blk_ordered_cur_seq(q))
                           *rqp = NULL;
           }
   
           return 1;
   }
   
   static void req_bio_endio(struct request *rq, struct bio *bio,
                             unsigned int nbytes, int error)
   {
           struct request_queue *q = rq->q;
   
           if (&q->bar_rq != rq) {
                   if (error)
                           clear_bit(BIO_UPTODATE, &bio->bi_flags);
                   else if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
                           error = -EIO;
   
                   if (unlikely(nbytes > bio->bi_size)) {
                           printk("%s: want %u bytes done, only %u left\n",
                                  __FUNCTION__, nbytes, bio->bi_size);
                           nbytes = bio->bi_size;
                   }
   
                   bio->bi_size -= nbytes;
                   bio->bi_sector += (nbytes >> 9);
                   if (bio->bi_size == 0)
                           bio_endio(bio, error);
           } else {
   
                   /*
                    * Okay, this is the barrier request in progress, just
                    * record the error;
                    */
                   if (error && !q->orderr)
                           q->orderr = error;
           }
   }
   
   /**
    * blk_queue_bounce_limit - set bounce buffer limit for queue
    * @q:  the request queue for the device
    * @dma_addr:   bus address limit
    *
    * Description:
    *    Different hardware can have different requirements as to what pages
    *    it can do I/O directly to. A low level driver can call
    *    blk_queue_bounce_limit to have lower memory pages allocated as bounce
    *    buffers for doing I/O to pages residing above @page.
    **/
   void blk_queue_bounce_limit(struct request_queue *q, u64 dma_addr)
   {
           unsigned long bounce_pfn = dma_addr >> PAGE_SHIFT;
           int dma = 0;
   
           q->bounce_gfp = GFP_NOIO;
   #if BITS_PER_LONG == 64
           /* Assume anything <= 4GB can be handled by IOMMU.
              Actually some IOMMUs can handle everything, but I don't
              know of a way to test this here. */
           if (bounce_pfn < (min_t(u64,0xffffffff,BLK_BOUNCE_HIGH) >> PAGE_SHIFT))
                   dma = 1;
           q->bounce_pfn = max_low_pfn;
   #else
           if (bounce_pfn < blk_max_low_pfn)
                   dma = 1;
           q->bounce_pfn = bounce_pfn;
   #endif
           if (dma) {
                   init_emergency_isa_pool();
                   q->bounce_gfp = GFP_NOIO | GFP_DMA;
                   q->bounce_pfn = bounce_pfn;
           }
   }
   
   EXPORT_SYMBOL(blk_queue_bounce_limit);
   
   /**
    * blk_queue_max_sectors - set max sectors for a request for this queue
    * @q:  the request queue for the device
    * @max_sectors:  max sectors in the usual 512b unit
    *
    * Description:
    *    Enables a low level driver to set an upper limit on the size of
    *    received requests.
    **/
   void blk_queue_max_sectors(struct request_queue *q, unsigned int max_sectors)
   {
           if ((max_sectors << 9) < PAGE_CACHE_SIZE) {
                   max_sectors = 1 << (PAGE_CACHE_SHIFT - 9);
                   printk("%s: set to minimum %d\n", __FUNCTION__, max_sectors);
           }
   
           if (BLK_DEF_MAX_SECTORS > max_sectors)
                   q->max_hw_sectors = q->max_sectors = max_sectors;
           else {
                   q->max_sectors = BLK_DEF_MAX_SECTORS;
                   q->max_hw_sectors = max_sectors;
           }
   }
   
   EXPORT_SYMBOL(blk_queue_max_sectors);
   
   /**
    * blk_queue_max_phys_segments - set max phys segments for a request for this queue
    * @q:  the request queue for the device
    * @max_segments:  max number of segments
    *
    * Description:
    *    Enables a low level driver to set an upper limit on the number of
    *    physical data segments in a request.  This would be the largest sized
    *    scatter list the driver could handle.
    **/
   void blk_queue_max_phys_segments(struct request_queue *q,
                                    unsigned short max_segments)
   {
           if (!max_segments) {
                   max_segments = 1;
                   printk("%s: set to minimum %d\n", __FUNCTION__, max_segments);
           }
   
           q->max_phys_segments = max_segments;
   }
   
   EXPORT_SYMBOL(blk_queue_max_phys_segments);
   
   /**
    * blk_queue_max_hw_segments - set max hw segments for a request for this queue
    * @q:  the request queue for the device
    * @max_segments:  max number of segments
    *
    * Description:
    *    Enables a low level driver to set an upper limit on the number of
    *    hw data segments in a request.  This would be the largest number of
    *    address/length pairs the host adapter can actually give as once
    *    to the device.
    **/
   void blk_queue_max_hw_segments(struct request_queue *q,
                                  unsigned short max_segments)
   {
           if (!max_segments) {
                   max_segments = 1;
                   printk("%s: set to minimum %d\n", __FUNCTION__, max_segments);
           }
   
           q->max_hw_segments = max_segments;
   }
   
   EXPORT_SYMBOL(blk_queue_max_hw_segments);
   
   /**
    * blk_queue_max_segment_size - set max segment size for blk_rq_map_sg
    * @q:  the request queue for the device
    * @max_size:  max size of segment in bytes
    *
    * Description:
    *    Enables a low level driver to set an upper limit on the size of a
    *    coalesced segment
    **/
   void blk_queue_max_segment_size(struct request_queue *q, unsigned int max_size)
   {
           if (max_size < PAGE_CACHE_SIZE) {
                   max_size = PAGE_CACHE_SIZE;
                   printk("%s: set to minimum %d\n", __FUNCTION__, max_size);
           }
   
           q->max_segment_size = max_size;
   }
   
   EXPORT_SYMBOL(blk_queue_max_segment_size);
   
   /**
    * blk_queue_hardsect_size - set hardware sector size for the queue
    * @q:  the request queue for the device
    * @size:  the hardware sector size, in bytes
    *
    * Description:
    *   This should typically be set to the lowest possible sector size
    *   that the hardware can operate on (possible without reverting to
    *   even internal read-modify-write operations). Usually the default
    *   of 512 covers most hardware.
    **/
   void blk_queue_hardsect_size(struct request_queue *q, unsigned short size)
   {
           q->hardsect_size = size;
   }
   
   EXPORT_SYMBOL(blk_queue_hardsect_size);
   
   /*
    * Returns the minimum that is _not_ zero, unless both are zero.
    */
   #define min_not_zero(l, r) (l == 0) ? r : ((r == 0) ? l : min(l, r))
   
   /**
    * blk_queue_stack_limits - inherit underlying queue limits for stacked drivers
    * @t:  the stacking driver (top)
    * @b:  the underlying device (bottom)
    **/
   void blk_queue_stack_limits(struct request_queue *t, struct request_queue *b)
   {
           /* zero is "infinity" */
           t->max_sectors = min_not_zero(t->max_sectors,b->max_sectors);
           t->max_hw_sectors = min_not_zero(t->max_hw_sectors,b->max_hw_sectors);
   
           t->max_phys_segments = min(t->max_phys_segments,b->max_phys_segments);
           t->max_hw_segments = min(t->max_hw_segments,b->max_hw_segments);
           t->max_segment_size = min(t->max_segment_size,b->max_segment_size);
           t->hardsect_size = max(t->hardsect_size,b->hardsect_size);
           if (!test_bit(QUEUE_FLAG_CLUSTER, &b->queue_flags))
                   clear_bit(QUEUE_FLAG_CLUSTER, &t->queue_flags);
   }
   
   EXPORT_SYMBOL(blk_queue_stack_limits);
   
   /**
    * blk_queue_segment_boundary - set boundary rules for segment merging
    * @q:  the request queue for the device
    * @mask:  the memory boundary mask
    **/
   void blk_queue_segment_boundary(struct request_queue *q, unsigned long mask)
   {
           if (mask < PAGE_CACHE_SIZE - 1) {
                   mask = PAGE_CACHE_SIZE - 1;
                   printk("%s: set to minimum %lx\n", __FUNCTION__, mask);
           }
   
           q->seg_boundary_mask = mask;
   }
   
   EXPORT_SYMBOL(blk_queue_segment_boundary);
   
   /**
    * blk_queue_dma_alignment - set dma length and memory alignment
    * @q:     the request queue for the device
    * @mask:  alignment mask
    *
    * description:
    *    set required memory and length aligment for direct dma transactions.
    *    this is used when buiding direct io requests for the queue.
    *
    **/
   void blk_queue_dma_alignment(struct request_queue *q, int mask)
   {
           q->dma_alignment = mask;
   }
   
   EXPORT_SYMBOL(blk_queue_dma_alignment);
   
   /**
    * blk_queue_update_dma_alignment - update dma length and memory alignment
    * @q:     the request queue for the device
    * @mask:  alignment mask
    *
    * description:
    *    update required memory and length aligment for direct dma transactions.
    *    If the requested alignment is larger than the current alignment, then
    *    the current queue alignment is updated to the new value, otherwise it
    *    is left alone.  The design of this is to allow multiple objects
    *    (driver, device, transport etc) to set their respective
    *    alignments without having them interfere.
    *
    **/
   void blk_queue_update_dma_alignment(struct request_queue *q, int mask)
   {
           BUG_ON(mask > PAGE_SIZE);
   
           if (mask > q->dma_alignment)
                   q->dma_alignment = mask;
   }
   
   EXPORT_SYMBOL(blk_queue_update_dma_alignment);
   
   /**
    * blk_queue_find_tag - find a request by its tag and queue
    * @q:   The request queue for the device
    * @tag: The tag of the request
    *
    * Notes:
    *    Should be used when a device returns a tag and you want to match
    *    it with a request.
    *
    *    no locks need be held.
    **/
   struct request *blk_queue_find_tag(struct request_queue *q, int tag)
   {
           return blk_map_queue_find_tag(q->queue_tags, tag);
   }
   
   EXPORT_SYMBOL(blk_queue_find_tag);
   
   /**
    * __blk_free_tags - release a given set of tag maintenance info
    * @bqt:        the tag map to free
    *
    * Tries to free the specified @bqt@.  Returns true if it was
    * actually freed and false if there are still references using it
    */
   static int __blk_free_tags(struct blk_queue_tag *bqt)
   {
           int retval;
   
           retval = atomic_dec_and_test(&bqt->refcnt);
           if (retval) {
                   BUG_ON(bqt->busy);
   
                   kfree(bqt->tag_index);
                   bqt->tag_index = NULL;
   
                   kfree(bqt->tag_map);
                   bqt->tag_map = NULL;
   
                   kfree(bqt);
   
           }
   
           return retval;
   }
   
   /**
    * __blk_queue_free_tags - release tag maintenance info
    * @q:  the request queue for the device
    *
    *  Notes:
    *    blk_cleanup_queue() will take care of calling this function, if tagging
    *    has been used. So there's no need to call this directly.
    **/
   static void __blk_queue_free_tags(struct request_queue *q)
   {
           struct blk_queue_tag *bqt = q->queue_tags;
   
           if (!bqt)
                   return;
   
           __blk_free_tags(bqt);
   
           q->queue_tags = NULL;
           q->queue_flags &= ~(1 << QUEUE_FLAG_QUEUED);
   }
   
   
   /**
    * blk_free_tags - release a given set of tag maintenance info
    * @bqt:        the tag map to free
    *
    * For externally managed @bqt@ frees the map.  Callers of this
    * function must guarantee to have released all the queues that
    * might have been using this tag map.
    */
   void blk_free_tags(struct blk_queue_tag *bqt)
   {
           if (unlikely(!__blk_free_tags(bqt)))
                   BUG();
   }
   EXPORT_SYMBOL(blk_free_tags);
   
   /**
    * blk_queue_free_tags - release tag maintenance info
    * @q:  the request queue for the device
    *
    *  Notes:
    *      This is used to disabled tagged queuing to a device, yet leave
    *      queue in function.
    **/
   void blk_queue_free_tags(struct request_queue *q)
   {
           clear_bit(QUEUE_FLAG_QUEUED, &q->queue_flags);
   }
   
   EXPORT_SYMBOL(blk_queue_free_tags);
   
   static int
   init_tag_map(struct request_queue *q, struct blk_queue_tag *tags, int depth)
   {
           struct request **tag_index;
           unsigned long *tag_map;
           int nr_ulongs;
   
           if (q && depth > q->nr_requests * 2) {
                   depth = q->nr_requests * 2;
                   printk(KERN_ERR "%s: adjusted depth to %d\n",
                                   __FUNCTION__, depth);
           }
   
           tag_index = kzalloc(depth * sizeof(struct request *), GFP_ATOMIC);
           if (!tag_index)
                   goto fail;
   
           nr_ulongs = ALIGN(depth, BITS_PER_LONG) / BITS_PER_LONG;
           tag_map = kzalloc(nr_ulongs * sizeof(unsigned long), GFP_ATOMIC);
           if (!tag_map)
                   goto fail;
   
           tags->real_max_depth = depth;
           tags->max_depth = depth;
           tags->tag_index = tag_index;
           tags->tag_map = tag_map;
   
           return 0;
   fail:
           kfree(tag_index);
           return -ENOMEM;
   }
   
   static struct blk_queue_tag *__blk_queue_init_tags(struct request_queue *q,
                                                      int depth)
   {
           struct blk_queue_tag *tags;
   
           tags = kmalloc(sizeof(struct blk_queue_tag), GFP_ATOMIC);
           if (!tags)
                   goto fail;
   
           if (init_tag_map(q, tags, depth))
                   goto fail;
   
           tags->busy = 0;
           atomic_set(&tags->refcnt, 1);
           return tags;
   fail:
           kfree(tags);
           return NULL;
   }
   
   /**
    * blk_init_tags - initialize the tag info for an external tag map
    * @depth:      the maximum queue depth supported
    * @tags: the tag to use
    **/
   struct blk_queue_tag *blk_init_tags(int depth)
   {
           return __blk_queue_init_tags(NULL, depth);
   }
   EXPORT_SYMBOL(blk_init_tags);
   
   /**
    * blk_queue_init_tags - initialize the queue tag info
    * @q:  the request queue for the device
    * @depth:  the maximum queue depth supported
    * @tags: the tag to use
    **/
   int blk_queue_init_tags(struct request_queue *q, int depth,
                           struct blk_queue_tag *tags)
   {
           int rc;
   
           BUG_ON(tags && q->queue_tags && tags != q->queue_tags);
   
           if (!tags && !q->queue_tags) {
                   tags = __blk_queue_init_tags(q, depth);
   
                   if (!tags)
                           goto fail;
           } else if (q->queue_tags) {
                   if ((rc = blk_queue_resize_tags(q, depth)))
                           return rc;
                   set_bit(QUEUE_FLAG_QUEUED, &q->queue_flags);
                   return 0;
           } else
                   atomic_inc(&tags->refcnt);
   
           /*
            * assign it, all done
            */
           q->queue_tags = tags;
           q->queue_flags |= (1 << QUEUE_FLAG_QUEUED);
           INIT_LIST_HEAD(&q->tag_busy_list);
           return 0;
   fail:
           kfree(tags);
           return -ENOMEM;
   }
   
   EXPORT_SYMBOL(blk_queue_init_tags);
   
   /**
    * blk_queue_resize_tags - change the queueing depth
    * @q:  the request queue for the device
    * @new_depth: the new max command queueing depth
    *
    *  Notes:
    *    Must be called with the queue lock held.
    **/
   int blk_queue_resize_tags(struct request_queue *q, int new_depth)
   {
           struct blk_queue_tag *bqt = q->queue_tags;
           struct request **tag_index;
          unsigned long *tag_map;
          int max_depth, nr_ulongs;
  
          if (!bqt)
                  return -ENXIO;
  
          /*
           * if we already have large enough real_max_depth.  just
           * adjust max_depth.  *NOTE* as requests with tag value
           * between new_depth and real_max_depth can be in-flight, tag
           * map can not be shrunk blindly here.
           */
          if (new_depth <= bqt->real_max_depth) {
                  bqt->max_depth = new_depth;
                  return 0;
          }
  
          /*
           * Currently cannot replace a shared tag map with a new
           * one, so error out if this is the case
           */
          if (atomic_read(&bqt->refcnt) != 1)
                  return -EBUSY;
  
          /*
           * save the old state info, so we can copy it back
           */
          tag_index = bqt->tag_index;
          tag_map = bqt->tag_map;
          max_depth = bqt->real_max_depth;
  
          if (init_tag_map(q, bqt, new_depth))
                  return -ENOMEM;
  
          memcpy(bqt->tag_index, tag_index, max_depth * sizeof(struct request *));
          nr_ulongs = ALIGN(max_depth, BITS_PER_LONG) / BITS_PER_LONG;
          memcpy(bqt->tag_map, tag_map, nr_ulongs * sizeof(unsigned long));
  
          kfree(tag_index);
          kfree(tag_map);
          return 0;
  }
  
  EXPORT_SYMBOL(blk_queue_resize_tags);
  
  /**
   * blk_queue_end_tag - end tag operations for a request
   * @q:  the request queue for the device
   * @rq: the request that has completed
   *
   *  Description:
   *    Typically called when end_that_request_first() returns 0, meaning
   *    all transfers have been done for a request. It's important to call
   *    this function before end_that_request_last(), as that will put the
   *    request back on the free list thus corrupting the internal tag list.
   *
   *  Notes:
   *   queue lock must be held.
   **/
  void blk_queue_end_tag(struct request_queue *q, struct request *rq)
  {
          struct blk_queue_tag *bqt = q->queue_tags;
          int tag = rq->tag;
  
          BUG_ON(tag == -1);
  
          if (unlikely(tag >= bqt->real_max_depth))
                  /*
                   * This can happen after tag depth has been reduced.
                   * FIXME: how about a warning or info message here?
                   */
                  return;
  
          list_del_init(&rq->queuelist);
          rq->cmd_flags &= ~REQ_QUEUED;
          rq->tag = -1;
  
          if (unlikely(bqt->tag_index[tag] == NULL))
                  printk(KERN_ERR "%s: tag %d is missing\n",
                         __FUNCTION__, tag);
  
          bqt->tag_index[tag] = NULL;
  
          if (unlikely(!test_bit(tag, bqt->tag_map))) {
                  printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
                         __FUNCTION__, tag);
                  return;
          }
          /*
           * The tag_map bit acts as a lock for tag_index[bit], so we need
           * unlock memory barrier semantics.
           */
          clear_bit_unlock(tag, bqt->tag_map);
          bqt->busy--;
  }
  
  EXPORT_SYMBOL(blk_queue_end_tag);
  
  /**
   * blk_queue_start_tag - find a free tag and assign it
   * @q:  the request queue for the device
   * @rq:  the block request that needs tagging
   *
   *  Description:
   *    This can either be used as a stand-alone helper, or possibly be
   *    assigned as the queue &prep_rq_fn (in which case &struct request
   *    automagically gets a tag assigned). Note that this function
   *    assumes that any type of request can be queued! if this is not
   *    true for your device, you must check the request type before
   *    calling this function.  The request will also be removed from
   *    the request queue, so it's the drivers responsibility to readd
   *    it if it should need to be restarted for some reason.
   *
   *  Notes:
   *   queue lock must be held.
   **/
  int blk_queue_start_tag(struct request_queue *q, struct request *rq)
  {
          struct blk_queue_tag *bqt = q->queue_tags;
          int tag;
  
          if (unlikely((rq->cmd_flags & REQ_QUEUED))) {
                  printk(KERN_ERR 
                         "%s: request %p for device [%s] already tagged %d",
                         __FUNCTION__, rq,
                         rq->rq_disk ? rq->rq_disk->disk_name : "?", rq->tag);
                  BUG();
          }
  
          /*
           * Protect against shared tag maps, as we may not have exclusive
           * access to the tag map.
           */
          do {
                  tag = find_first_zero_bit(bqt->tag_map, bqt->max_depth);
                  if (tag >= bqt->max_depth)
                          return 1;
  
          } while (test_and_set_bit_lock(tag, bqt->tag_map));
          /*
           * We need lock ordering semantics given by test_and_set_bit_lock.
           * See blk_queue_end_tag for details.
           */
  
          rq->cmd_flags |= REQ_QUEUED;
          rq->tag = tag;
          bqt->tag_index[tag] = rq;
          blkdev_dequeue_request(rq);
          list_add(&rq->queuelist, &q->tag_busy_list);
          bqt->busy++;
          return 0;
  }
  
  EXPORT_SYMBOL(blk_queue_start_tag);
  
  /**
   * blk_queue_invalidate_tags - invalidate all pending tags
   * @q:  the request queue for the device
   *
   *  Description:
   *   Hardware conditions may dictate a need to stop all pending requests.
   *   In this case, we will safely clear the block side of the tag queue and
   *   readd all requests to the request queue in the right order.
   *
   *  Notes:
   *   queue lock must be held.
   **/
  void blk_queue_invalidate_tags(struct request_queue *q)
  {
          struct list_head *tmp, *n;
  
          list_for_each_safe(tmp, n, &q->tag_busy_list)
                  blk_requeue_request(q, list_entry_rq(tmp));
  }
  
  EXPORT_SYMBOL(blk_queue_invalidate_tags);
  
  void blk_dump_rq_flags(struct request *rq, char *msg)
  {
          int bit;
  
          printk("%s: dev %s: type=%x, flags=%x\n", msg,
                  rq->rq_disk ? rq->rq_disk->disk_name : "?", rq->cmd_type,
                  rq->cmd_flags);
  
          printk("\nsector %llu, nr/cnr %lu/%u\n", (unsigned long long)rq->sector,
                                                         rq->nr_sectors,
                                                         rq->current_nr_sectors);
          printk("bio %p, biotail %p, buffer %p, data %p, len %u\n", rq->bio, rq->biotail, rq->buffer, rq->data, rq->data_len);
  
          if (blk_pc_request(rq)) {
                  printk("cdb: ");
                  for (bit = 0; bit < sizeof(rq->cmd); bit++)
                          printk("%02x ", rq->cmd[bit]);
                  printk("\n");
          }
  }
  
  EXPORT_SYMBOL(blk_dump_rq_flags);
  
  void blk_recount_segments(struct request_queue *q, struct bio *bio)
  {
          struct request rq;
          struct bio *nxt = bio->bi_next;
          rq.q = q;
          rq.bio = rq.biotail = bio;
          bio->bi_next = NULL;
          blk_recalc_rq_segments(&rq);
          bio->bi_next = nxt;
          bio->bi_phys_segments = rq.nr_phys_segments;
          bio->bi_hw_segments = rq.nr_hw_segments;
          bio->bi_flags |= (1 << BIO_SEG_VALID);
  }
  EXPORT_SYMBOL(blk_recount_segments);
  
  static void blk_recalc_rq_segments(struct request *rq)
  {
          int nr_phys_segs;
          int nr_hw_segs;
          unsigned int phys_size;
          unsigned int hw_size;
          struct bio_vec *bv, *bvprv = NULL;
          int seg_size;
          int hw_seg_size;
          int cluster;
          struct req_iterator iter;
          int high, highprv = 1;
          struct request_queue *q = rq->q;
  
          if (!rq->bio)
                  return;
  
          cluster = q->queue_flags & (1 << QUEUE_FLAG_CLUSTER);
          hw_seg_size = seg_size = 0;
          phys_size = hw_size = nr_phys_segs = nr_hw_segs = 0;
          rq_for_each_segment(bv, rq, iter) {
                  /*
                   * the trick here is making sure that a high page is never
                   * considered part of another segment, since that might
                   * change with the bounce page.
                   */
                  high = page_to_pfn(bv->bv_page) > q->bounce_pfn;
                  if (high || highprv)
                          goto new_hw_segment;
                  if (cluster) {
                          if (seg_size + bv->bv_len > q->max_segment_size)
                                  goto new_segment;
                          if (!BIOVEC_PHYS_MERGEABLE(bvprv, bv))
                                  goto new_segment;
                          if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bv))
                                  goto new_segment;
                          if (BIOVEC_VIRT_OVERSIZE(hw_seg_size + bv->bv_len))
                                  goto new_hw_segment;
  
                          seg_size += bv->bv_len;
                          hw_seg_size += bv->bv_len;
                          bvprv = bv;
                          continue;
                  }
  new_segment:
                  if (BIOVEC_VIRT_MERGEABLE(bvprv, bv) &&
                      !BIOVEC_VIRT_OVERSIZE(hw_seg_size + bv->bv_len))
                          hw_seg_size += bv->bv_len;
                  else {
  new_hw_segment:
                          if (nr_hw_segs == 1 &&
                              hw_seg_size > rq->bio->bi_hw_front_size)
                                  rq->bio->bi_hw_front_size = hw_seg_size;
                          hw_seg_size = BIOVEC_VIRT_START_SIZE(bv) + bv->bv_len;
                          nr_hw_segs++;
                  }
  
                  nr_phys_segs++;
                  bvprv = bv;
                  seg_size = bv->bv_len;
                  highprv = high;
          }
  
          if (nr_hw_segs == 1 &&
              hw_seg_size > rq->bio->bi_hw_front_size)
                  rq->bio->bi_hw_front_size = hw_seg_size;
          if (hw_seg_size > rq->biotail->bi_hw_back_size)
                  rq->biotail->bi_hw_back_size = hw_seg_size;
          rq->nr_phys_segments = nr_phys_segs;
          rq->nr_hw_segments = nr_hw_segs;
  }
  
  static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
                                     struct bio *nxt)
  {
          if (!(q->queue_flags & (1 << QUEUE_FLAG_CLUSTER)))
                  return 0;
  
          if (!BIOVEC_PHYS_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)))
                  return 0;
          if (bio->bi_size + nxt->bi_size > q->max_segment_size)
                  return 0;
  
          /*
           * bio and nxt are contigous in memory, check if the queue allows
           * these two to be merged into one
           */
          if (BIO_SEG_BOUNDARY(q, bio, nxt))
                  return 1;
  
          return 0;
  }
  
  static int blk_hw_contig_segment(struct request_queue *q, struct bio *bio,
                                   struct bio *nxt)
  {
          if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
                  blk_recount_segments(q, bio);
          if (unlikely(!bio_flagged(nxt, BIO_SEG_VALID)))
                  blk_recount_segments(q, nxt);
          if (!BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)) ||
              BIOVEC_VIRT_OVERSIZE(bio->bi_hw_back_size + nxt->bi_hw_front_size))
                  return 0;
          if (bio->bi_hw_back_size + nxt->bi_hw_front_size > q->max_segment_size)
                  return 0;
  
          return 1;
  }
  
  /*
   * map a request to scatterlist, return number of sg entries setup. Caller
   * must make sure sg can hold rq->nr_phys_segments entries
   */
  int blk_rq_map_sg(struct request_queue *q, struct request *rq,
                    struct scatterlist *sglist)
  {
          struct bio_vec *bvec, *bvprv;
          struct req_iterator iter;
          struct scatterlist *sg;
          int nsegs, cluster;
  
          nsegs = 0;
          cluster = q->queue_flags & (1 << QUEUE_FLAG_CLUSTER);
  
          /*
           * for each bio in rq
           */
          bvprv = NULL;
          sg = NULL;
          rq_for_each_segment(bvec, rq, iter) {
                  int nbytes = bvec->bv_len;
  
                  if (bvprv && cluster) {
                          if (sg->length + nbytes > q->max_segment_size)
                                  goto new_segment;
  
                          if (!BIOVEC_PHYS_MERGEABLE(bvprv, bvec))
                                  goto new_segment;
                          if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bvec))
                                  goto new_segment;
  
                          sg->length += nbytes;
                  } else {
  new_segment:
                          if (!sg)
                                  sg = sglist;
                          else {
                                  /*
                                   * If the driver previously mapped a shorter
                                   * list, we could see a termination bit
                                   * prematurely unless it fully inits the sg
                                   * table on each mapping. We KNOW that there
                                   * must be more entries here or the driver
                                   * would be buggy, so force clear the
                                   * termination bit to avoid doing a full
                                   * sg_init_table() in drivers for each command.
                                   */
                                  sg->page_link &= ~0x02;
                                  sg = sg_next(sg);
                          }
  
                          sg_set_page(sg, bvec->bv_page, nbytes, bvec->bv_offset);
                          nsegs++;
                  }
                  bvprv = bvec;
          } /* segments in rq */
  
          if (sg)
                  sg_mark_end(sg);
  
          return nsegs;
  }
  
  EXPORT_SYMBOL(blk_rq_map_sg);
  
  /*
   * the standard queue merge functions, can be overridden with device
   * specific ones if so desired
   */
  
  static inline int ll_new_mergeable(struct request_queue *q,
                                     struct request *req,
                                     struct bio *bio)
  {
          int nr_phys_segs = bio_phys_segments(q, bio);
  
          if (req->nr_phys_segments + nr_phys_segs > q->max_phys_segments) {
                  req->cmd_flags |= REQ_NOMERGE;
                  if (req == q->last_merge)
                          q->last_merge = NULL;
                  return 0;
          }
  
          /*
           * A hw segment is just getting larger, bump just the phys
           * counter.
           */
          req->nr_phys_segments += nr_phys_segs;
          return 1;
  }
  
  static inline int ll_new_hw_segment(struct request_queue *q,
                                      struct request *req,
                                      struct bio *bio)
  {
          int nr_hw_segs = bio_hw_segments(q, bio);
          int nr_phys_segs = bio_phys_segments(q, bio);
  
          if (req->nr_hw_segments + nr_hw_segs > q->max_hw_segments
              || req->nr_phys_segments + nr_phys_segs > q->max_phys_segments) {
                  req->cmd_flags |= REQ_NOMERGE;
                  if (req == q->last_merge)
                          q->last_merge = NULL;
                  return 0;
          }
  
          /*
           * This will form the start of a new hw segment.  Bump both
           * counters.
           */
          req->nr_hw_segments += nr_hw_segs;
          req->nr_phys_segments += nr_phys_segs;
          return 1;
  }
  
  static int ll_back_merge_fn(struct request_queue *q, struct request *req,
                              struct bio *bio)
  {
          unsigned short max_sectors;
          int len;
  
          if (unlikely(blk_pc_request(req)))
                  max_sectors = q->max_hw_sectors;
          else
                  max_sectors = q->max_sectors;
  
          if (req->nr_sectors + bio_sectors(bio) > max_sectors) {
                  req->cmd_flags |= REQ_NOMERGE;
                  if (req == q->last_merge)
                          q->last_merge = NULL;
                  return 0;
          }
          if (unlikely(!bio_flagged(req->biotail, BIO_SEG_VALID)))
                  blk_recount_segments(q, req->biotail);
          if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
                  blk_recount_segments(q, bio);
          len = req->biotail->bi_hw_back_size + bio->bi_hw_front_size;
          if (BIOVEC_VIRT_MERGEABLE(__BVEC_END(req->biotail), __BVEC_START(bio)) &&
              !BIOVEC_VIRT_OVERSIZE(len)) {
                  int mergeable =  ll_new_mergeable(q, req, bio);
  
                  if (mergeable) {
                          if (req->nr_hw_segments == 1)
                                  req->bio->bi_hw_front_size = len;
                          if (bio->bi_hw_segments == 1)
                                  bio->bi_hw_back_size = len;
                  }
                  return mergeable;
          }
  
          return ll_new_hw_segment(q, req, bio);
  }
  
  static int ll_front_merge_fn(struct request_queue *q, struct request *req, 
                               struct bio *bio)
  {
          unsigned short max_sectors;
          int len;
  
          if (unlikely(blk_pc_request(req)))
                  max_sectors = q->max_hw_sectors;
          else
                  max_sectors = q->max_sectors;
  
  
          if (req->nr_sectors + bio_sectors(bio) > max_sectors) {
                  req->cmd_flags |= REQ_NOMERGE;
                  if (req == q->last_merge)
                          q->last_merge = NULL;
                  return 0;
          }
          len = bio->bi_hw_back_size + req->bio->bi_hw_front_size;
          if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
                  blk_recount_segments(q, bio);
          if (unlikely(!bio_flagged(req->bio, BIO_SEG_VALID)))
                  blk_recount_segments(q, req->bio);
          if (BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(req->bio)) &&
              !BIOVEC_VIRT_OVERSIZE(len)) {
                  int mergeable =  ll_new_mergeable(q, req, bio);
  
                  if (mergeable) {
                          if (bio->bi_hw_segments == 1)
                                  bio->bi_hw_front_size = len;
                          if (req->nr_hw_segments == 1)
                                  req->biotail->bi_hw_back_size = len;
                  }
                  return mergeable;
          }
  
          return ll_new_hw_segment(q, req, bio);
  }
  
  static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
                                  struct request *next)
  {
          int total_phys_segments;
          int total_hw_segments;
  
          /*
           * First check if the either of the requests are re-queued
           * requests.  Can't merge them if they are.
           */
          if (req->special || next->special)
                  return 0;
  
          /*
           * Will it become too large?
           */
          if ((req->nr_sectors + next->nr_sectors) > q->max_sectors)
                  return 0;
  
          total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
          if (blk_phys_contig_segment(q, req->biotail, next->bio))
                  total_phys_segments--;
  
          if (total_phys_segments > q->max_phys_segments)
                  return 0;
  
          total_hw_segments = req->nr_hw_segments + next->nr_hw_segments;
          if (blk_hw_contig_segment(q, req->biotail, next->bio)) {
                  int len = req->biotail->bi_hw_back_size + next->bio->bi_hw_front_size;
                  /*
                   * propagate the combined length to the end of the requests
                   */
                  if (req->nr_hw_segments == 1)
                          req->bio->bi_hw_front_size = len;
                  if (next->nr_hw_segments == 1)
                          next->biotail->bi_hw_back_size = len;
                  total_hw_segments--;
          }
  
          if (total_hw_segments > q->max_hw_segments)
                  return 0;
  
          /* Merge is OK... */
          req->nr_phys_segments = total_phys_segments;
          req->nr_hw_segments = total_hw_segments;
          return 1;
  }
  
  /*
   * "plug" the device if there are no outstanding requests: this will
   * force the transfer to start only after we have put all the requests
   * on the list.
   *
   * This is called with interrupts off and no requests on the queue and
   * with the queue lock held.
   */
  void blk_plug_device(struct request_queue *q)
  {
          WARN_ON(!irqs_disabled());
  
          /*
           * don't plug a stopped queue, it must be paired with blk_start_queue()
           * which will restart the queueing
           */
          if (blk_queue_stopped(q))
                  return;
  
          if (!test_and_set_bit(QUEUE_FLAG_PLUGGED, &q->queue_flags)) {
                  mod_timer(&q->unplug_timer, jiffies + q->unplug_delay);
                  blk_add_trace_generic(q, NULL, 0, BLK_TA_PLUG);
          }
  }
  
  EXPORT_SYMBOL(blk_plug_device);
  
  /*
   * remove the queue from the plugged list, if present. called with
   * queue lock held and interrupts disabled.
   */
  int blk_remove_plug(struct request_queue *q)
  {
          WARN_ON(!irqs_disabled());
  
          if (!test_and_clear_bit(QUEUE_FLAG_PLUGGED, &q->queue_flags))
                  return 0;
  
          del_timer(&q->unplug_timer);
          return 1;
  }
  
  EXPORT_SYMBOL(blk_remove_plug);
  
  /*
   * remove the plug and let it rip..
   */
  void __generic_unplug_device(struct request_queue *q)
  {
          if (unlikely(blk_queue_stopped(q)))
                  return;
  
          if (!blk_remove_plug(q))
                  return;
  
          q->request_fn(q);
  }
  EXPORT_SYMBOL(__generic_unplug_device);
  
  /**
   * generic_unplug_device - fire a request queue
   * @q:    The &struct request_queue in question
   *
   * Description:
   *   Linux uses plugging to build bigger requests queues before letting
   *   the device have at them. If a queue is plugged, the I/O scheduler
   *   is still adding and merging requests on the queue. Once the queue
   *   gets unplugged, the request_fn defined for the queue is invoked and
   *   transfers started.
   **/
  void generic_unplug_device(struct request_queue *q)
  {
          spin_lock_irq(q->queue_lock);
          __generic_unplug_device(q);
          spin_unlock_irq(q->queue_lock);
  }
  EXPORT_SYMBOL(generic_unplug_device);
  
  static void blk_backing_dev_unplug(struct backing_dev_info *bdi,
                                     struct page *page)
  {
          struct request_queue *q = bdi->unplug_io_data;
  
          blk_unplug(q);
  }
  
  static void blk_unplug_work(struct work_struct *work)
  {
          struct request_queue *q =
                  container_of(work, struct request_queue, unplug_work);
  
          blk_add_trace_pdu_int(q, BLK_TA_UNPLUG_IO, NULL,
                                  q->rq.count[READ] + q->rq.count[WRITE]);
  
          q->unplug_fn(q);
  }
  
  static void blk_unplug_timeout(unsigned long data)
  {
          struct request_queue *q = (struct request_queue *)data;
  
          blk_add_trace_pdu_int(q, BLK_TA_UNPLUG_TIMER, NULL,
                                  q->rq.count[READ] + q->rq.count[WRITE]);
  
          kblockd_schedule_work(&q->unplug_work);
  }
  
  void blk_unplug(struct request_queue *q)
  {
          /*
           * devices don't necessarily have an ->unplug_fn defined
           */
          if (q->unplug_fn) {
                  blk_add_trace_pdu_int(q, BLK_TA_UNPLUG_IO, NULL,
                                          q->rq.count[READ] + q->rq.count[WRITE]);
  
                  q->unplug_fn(q);
          }
  }
  EXPORT_SYMBOL(blk_unplug);
  
  /**
   * blk_start_queue - restart a previously stopped queue
   * @q:    The &struct request_queue in question
   *
   * Description:
   *   blk_start_queue() will clear the stop flag on the queue, and call
   *   the request_fn for the queue if it was in a stopped state when
   *   entered. Also see blk_stop_queue(). Queue lock must be held.
   **/
  void blk_start_queue(struct request_queue *q)
  {
          WARN_ON(!irqs_disabled());
  
          clear_bit(QUEUE_FLAG_STOPPED, &q->queue_flags);
  
          /*
           * one level of recursion is ok and is much faster than kicking
           * the unplug handling
           */
          if (!test_and_set_bit(QUEUE_FLAG_REENTER, &q->queue_flags)) {
                  q->request_fn(q);
                  clear_bit(QUEUE_FLAG_REENTER, &q->queue_flags);
          } else {
                  blk_plug_device(q);
                  kblockd_schedule_work(&q->unplug_work);
          }
  }
  
  EXPORT_SYMBOL(blk_start_queue);
  
  /**
   * blk_stop_queue - stop a queue
   * @q:    The &struct request_queue in question
   *
   * Description:
   *   The Linux block layer assumes that a block driver will consume all
   *   entries on the request queue when the request_fn strategy is called.
   *   Often this will not happen, because of hardware limitations (queue
   *   depth settings). If a device driver gets a 'queue full' response,
   *   or if it simply chooses not to queue more I/O at one point, it can
   *   call this function to prevent the request_fn from being called until
   *   the driver has signalled it's ready to go again. This happens by calling
   *   blk_start_queue() to restart queue operations. Queue lock must be held.
   **/
  void blk_stop_queue(struct request_queue *q)
  {
          blk_remove_plug(q);
          set_bit(QUEUE_FLAG_STOPPED, &q->queue_flags);
  }
  EXPORT_SYMBOL(blk_stop_queue);
  
  /**
   * blk_sync_queue - cancel any pending callbacks on a queue
   * @q: the queue
   *
   * Description:
   *     The block layer may perform asynchronous callback activity
   *     on a queue, such as calling the unplug function after a timeout.
   *     A block device may call blk_sync_queue to ensure that any
   *     such activity is cancelled, thus allowing it to release resources
   *     that the callbacks might use. The caller must already have made sure
   *     that its ->make_request_fn will not re-add plugging prior to calling
   *     this function.
   *
   */
  void blk_sync_queue(struct request_queue *q)
  {
          del_timer_sync(&q->unplug_timer);
          kblockd_flush_work(&q->unplug_work);
  }
  EXPORT_SYMBOL(blk_sync_queue);
  
  /**
   * blk_run_queue - run a single device queue
   * @q:  The queue to run
   */
  void blk_run_queue(struct request_queue *q)
  {
          unsigned long flags;
  
          spin_lock_irqsave(q->queue_lock, flags);
          blk_remove_plug(q);
  
          /*
           * Only recurse once to avoid overrunning the stack, let the unplug
           * handling reinvoke the handler shortly if we already got there.
           */
          if (!elv_queue_empty(q)) {
                  if (!test_and_set_bit(QUEUE_FLAG_REENTER, &q->queue_flags)) {
                          q->request_fn(q);
                          clear_bit(QUEUE_FLAG_REENTER, &q->queue_flags);
                  } else {
                          blk_plug_device(q);
                          kblockd_schedule_work(&q->unplug_work);
                  }
          }
  
          spin_unlock_irqrestore(q->queue_lock, flags);
  }
  EXPORT_SYMBOL(blk_run_queue);
  
  /**
   * blk_cleanup_queue: - release a &struct request_queue when it is no longer needed
   * @kobj:    the kobj belonging of the request queue to be released
   *
   * Description:
   *     blk_cleanup_queue is the pair to blk_init_queue() or
   *     blk_queue_make_request().  It should be called when a request queue is
   *     being released; typically when a block device is being de-registered.
   *     Currently, its primary task it to free all the &struct request
   *     structures that were allocated to the queue and the queue itself.
   *
   * Caveat:
   *     Hopefully the low level driver will have finished any
   *     outstanding requests first...
   **/
  static void blk_release_queue(struct kobject *kobj)
  {
          struct request_queue *q =
                  container_of(kobj, struct request_queue, kobj);
          struct request_list *rl = &q->rq;
  
          blk_sync_queue(q);
  
          if (rl->rq_pool)
                  mempool_destroy(rl->rq_pool);
  
          if (q->queue_tags)
                  __blk_queue_free_tags(q);
  
          blk_trace_shutdown(q);
  
          bdi_destroy(&q->backing_dev_info);
          kmem_cache_free(requestq_cachep, q);
  }
  
  void blk_put_queue(struct request_queue *q)
  {
          kobject_put(&q->kobj);
  }
  EXPORT_SYMBOL(blk_put_queue);
  
  void blk_cleanup_queue(struct request_queue * q)
  {
          mutex_lock(&q->sysfs_lock);
          set_bit(QUEUE_FLAG_DEAD, &q->queue_flags);
          mutex_unlock(&q->sysfs_lock);
  
          if (q->elevator)
                  elevator_exit(q->elevator);
  
          blk_put_queue(q);
  }
  
  EXPORT_SYMBOL(blk_cleanup_queue);
  
  static int blk_init_free_list(struct request_queue *q)
  {
          struct request_list *rl = &q->rq;
  
          rl->count[READ] = rl->count[WRITE] = 0;
          rl->starved[READ] = rl->starved[WRITE] = 0;
          rl->elvpriv = 0;
          init_waitqueue_head(&rl->wait[READ]);
          init_waitqueue_head(&rl->wait[WRITE]);
  
          rl->rq_pool = mempool_create_node(BLKDEV_MIN_RQ, mempool_alloc_slab,
                                  mempool_free_slab, request_cachep, q->node);
  
          if (!rl->rq_pool)
                  return -ENOMEM;
  
          return 0;
  }
  
  struct request_queue *blk_alloc_queue(gfp_t gfp_mask)
  {
          return blk_alloc_queue_node(gfp_mask, -1);
  }
  EXPORT_SYMBOL(blk_alloc_queue);
  
  static struct kobj_type queue_ktype;
  
  struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
  {
          struct request_queue *q;
          int err;
  
          q = kmem_cache_alloc_node(requestq_cachep,
                                  gfp_mask | __GFP_ZERO, node_id);
          if (!q)
                  return NULL;
  
          q->backing_dev_info.unplug_io_fn = blk_backing_dev_unplug;
          q->backing_dev_info.unplug_io_data = q;
          err = bdi_init(&q->backing_dev_info);
          if (err) {
                  kmem_cache_free(requestq_cachep, q);
                  return NULL;
          }
  
          init_timer(&q->unplug_timer);
  
          kobject_init(&q->kobj, &queue_ktype);
  
          mutex_init(&q->sysfs_lock);
  
          return q;
  }
  EXPORT_SYMBOL(blk_alloc_queue_node);
  
  /**
   * blk_init_queue  - prepare a request queue for use with a block device
   * @rfn:  The function to be called to process requests that have been
   *        placed on the queue.
   * @lock: Request queue spin lock
   *
   * Description:
   *    If a block device wishes to use the standard request handling procedures,
   *    which sorts requests and coalesces adjacent requests, then it must
   *    call blk_init_queue().  The function @rfn will be called when there
   *    are requests on the queue that need to be processed.  If the device
   *    supports plugging, then @rfn may not be called immediately when requests
   *    are available on the queue, but may be called at some time later instead.
   *    Plugged queues are generally unplugged when a buffer belonging to one
   *    of the requests on the queue is needed, or due to memory pressure.
   *
   *    @rfn is not required, or even expected, to remove all requests off the
   *    queue, but only as many as it can handle at a time.  If it does leave
   *    requests on the queue, it is responsible for arranging that the requests
   *    get dealt with eventually.
   *
   *    The queue spin lock must be held while manipulating the requests on the
   *    request queue; this lock will be taken also from interrupt context, so irq
   *    disabling is needed for it.
   *
   *    Function returns a pointer to the initialized request queue, or NULL if
   *    it didn't succeed.
   *
   * Note:
   *    blk_init_queue() must be paired with a blk_cleanup_queue() call
   *    when the block device is deactivated (such as at module unload).
   **/
  
  struct request_queue *blk_init_queue(request_fn_proc *rfn, spinlock_t *lock)
  {
          return blk_init_queue_node(rfn, lock, -1);
  }
  EXPORT_SYMBOL(blk_init_queue);
  
  struct request_queue *
  blk_init_queue_node(request_fn_proc *rfn, spinlock_t *lock, int node_id)
  {
          struct request_queue *q = blk_alloc_queue_node(GFP_KERNEL, node_id);
  
          if (!q)
                  return NULL;
  
          q->node = node_id;
          if (blk_init_free_list(q)) {
                  kmem_cache_free(requestq_cachep, q);
                  return NULL;
          }
  
          /*
           * if caller didn't supply a lock, they get per-queue locking with
           * our embedded lock
           */
          if (!lock) {
                  spin_lock_init(&q->__queue_lock);
                  lock = &q->__queue_lock;
          }
  
          q->request_fn           = rfn;
          q->prep_rq_fn           = NULL;
          q->unplug_fn            = generic_unplug_device;
          q->queue_flags          = (1 << QUEUE_FLAG_CLUSTER);
          q->queue_lock           = lock;
  
          blk_queue_segment_boundary(q, 0xffffffff);
  
          blk_queue_make_request(q, __make_request);
          blk_queue_max_segment_size(q, MAX_SEGMENT_SIZE);
  
          blk_queue_max_hw_segments(q, MAX_HW_SEGMENTS);
          blk_queue_max_phys_segments(q, MAX_PHYS_SEGMENTS);
  
          q->sg_reserved_size = INT_MAX;
  
          /*
           * all done
           */
          if (!elevator_init(q, NULL)) {
                  blk_queue_congestion_threshold(q);
                  return q;
          }
  
          blk_put_queue(q);
          return NULL;
  }
  EXPORT_SYMBOL(blk_init_queue_node);
  
  int blk_get_queue(struct request_queue *q)
  {
          if (likely(!test_bit(QUEUE_FLAG_DEAD, &q->queue_flags))) {
                  kobject_get(&q->kobj);
                  return 0;
          }
  
          return 1;
  }
  
  EXPORT_SYMBOL(blk_get_queue);
  
  static inline void blk_free_request(struct request_queue *q, struct request *rq)
  {
          if (rq->cmd_flags & REQ_ELVPRIV)
                  elv_put_request(q, rq);
          mempool_free(rq, q->rq.rq_pool);
  }
  
  static struct request *
  blk_alloc_request(struct request_queue *q, int rw, int priv, gfp_t gfp_mask)
  {
          struct request *rq = mempool_alloc(q->rq.rq_pool, gfp_mask);
  
          if (!rq)
                  return NULL;
  
          /*
           * first three bits are identical in rq->cmd_flags and bio->bi_rw,
           * see bio.h and blkdev.h
           */
          rq->cmd_flags = rw | REQ_ALLOCED;
  
          if (priv) {
                  if (unlikely(elv_set_request(q, rq, gfp_mask))) {
                          mempool_free(rq, q->rq.rq_pool);
                          return NULL;
                  }
                  rq->cmd_flags |= REQ_ELVPRIV;
          }
  
          return rq;
  }
  
  /*
   * ioc_batching returns true if the ioc is a valid batching request and
   * should be given priority access to a request.
   */
  static inline int ioc_batching(struct request_queue *q, struct io_context *ioc)
  {
          if (!ioc)
                  return 0;
  
          /*
           * Make sure the process is able to allocate at least 1 request
           * even if the batch times out, otherwise we could theoretically
           * lose wakeups.
           */
          return ioc->nr_batch_requests == q->nr_batching ||
                  (ioc->nr_batch_requests > 0
                  && time_before(jiffies, ioc->last_waited + BLK_BATCH_TIME));
  }
  
  /*
   * ioc_set_batching sets ioc to be a new "batcher" if it is not one. This
   * will cause the process to be a "batcher" on all queues in the system. This
   * is the behaviour we want though - once it gets a wakeup it should be given
   * a nice run.
   */
  static void ioc_set_batching(struct request_queue *q, struct io_context *ioc)
  {
          if (!ioc || ioc_batching(q, ioc))
                  return;
  
          ioc->nr_batch_requests = q->nr_batching;
          ioc->last_waited = jiffies;
  }
  
  static void __freed_request(struct request_queue *q, int rw)
  {
          struct request_list *rl = &q->rq;
  
          if (rl->count[rw] < queue_congestion_off_threshold(q))
                  blk_clear_queue_congested(q, rw);
  
          if (rl->count[rw] + 1 <= q->nr_requests) {
                  if (waitqueue_active(&rl->wait[rw]))
                          wake_up(&rl->wait[rw]);
  
                  blk_clear_queue_full(q, rw);
          }
  }
  
  /*
   * A request has just been released.  Account for it, update the full and
   * congestion status, wake up any waiters.   Called under q->queue_lock.
   */
  static void freed_request(struct request_queue *q, int rw, int priv)
  {
          struct request_list *rl = &q->rq;
  
          rl->count[rw]--;
          if (priv)
                  rl->elvpriv--;
  
          __freed_request(q, rw);
  
          if (unlikely(rl->starved[rw ^ 1]))
                  __freed_request(q, rw ^ 1);
  }
  
  #define blkdev_free_rq(list) list_entry((list)->next, struct request, queuelist)
  /*
   * Get a free request, queue_lock must be held.
   * Returns NULL on failure, with queue_lock held.
   * Returns !NULL on success, with queue_lock *not held*.
   */
  static struct request *get_request(struct request_queue *q, int rw_flags,
                                     struct bio *bio, gfp_t gfp_mask)
  {
          struct request *rq = NULL;
          struct request_list *rl = &q->rq;
          struct io_context *ioc = NULL;
          const int rw = rw_flags & 0x01;
          int may_queue, priv;
  
          may_queue = elv_may_queue(q, rw_flags);
          if (may_queue == ELV_MQUEUE_NO)
                  goto rq_starved;
  
          if (rl->count[rw]+1 >= queue_congestion_on_threshold(q)) {
                  if (rl->count[rw]+1 >= q->nr_requests) {
                          ioc = current_io_context(GFP_ATOMIC, q->node);
                          /*
                           * The queue will fill after this allocation, so set
                           * it as full, and mark this process as "batching".
                           * This process will be allowed to complete a batch of
                           * requests, others will be blocked.
                           */
                          if (!blk_queue_full(q, rw)) {
                                  ioc_set_batching(q, ioc);
                                  blk_set_queue_full(q, rw);
                          } else {
                                  if (may_queue != ELV_MQUEUE_MUST
                                                  && !ioc_batching(q, ioc)) {
                                          /*
                                           * The queue is full and the allocating
                                           * process is not a "batcher", and not
                                           * exempted by the IO scheduler
                                           */
                                          goto out;
                                  }
                          }
                  }
                  blk_set_queue_congested(q, rw);
          }
  
          /*
           * Only allow batching queuers to allocate up to 50% over the defined
           * limit of requests, otherwise we could have thousands of requests
           * allocated with any setting of ->nr_requests
           */
          if (rl->count[rw] >= (3 * q->nr_requests / 2))
                  goto out;
  
          rl->count[rw]++;
          rl->starved[rw] = 0;
  
          priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
          if (priv)
                  rl->elvpriv++;
  
          spin_unlock_irq(q->queue_lock);
  
          rq = blk_alloc_request(q, rw_flags, priv, gfp_mask);
          if (unlikely(!rq)) {
                  /*
                   * Allocation failed presumably due to memory. Undo anything
                   * we might have messed up.
                   *
                   * Allocating task should really be put onto the front of the
                   * wait queue, but this is pretty rare.
                   */
                  spin_lock_irq(q->queue_lock);
                  freed_request(q, rw, priv);
  
                  /*
                   * in the very unlikely event that allocation failed and no
                   * requests for this direction was pending, mark us starved
                   * so that freeing of a request in the other direction will
                   * notice us. another possible fix would be to split the
                   * rq mempool into READ and WRITE
                   */
  rq_starved:
                  if (unlikely(rl->count[rw] == 0))
                          rl->starved[rw] = 1;
  
                  goto out;
          }
  
          /*
           * ioc may be NULL here, and ioc_batching will be false. That's
           * OK, if the queue is under the request limit then requests need
           * not count toward the nr_batch_requests limit. There will always
           * be some limit enforced by BLK_BATCH_TIME.
           */
          if (ioc_batching(q, ioc))
                  ioc->nr_batch_requests--;
          
          rq_init(q, rq);
  
          blk_add_trace_generic(q, bio, rw, BLK_TA_GETRQ);
  out:
          return rq;
  }
  
  /*
   * No available requests for this queue, unplug the device and wait for some
   * requests to become available.
   *
   * Called with q->queue_lock held, and returns with it unlocked.
   */
  static struct request *get_request_wait(struct request_queue *q, int rw_flags,
                                          struct bio *bio)
  {
          const int rw = rw_flags & 0x01;
          struct request *rq;
  
          rq = get_request(q, rw_flags, bio, GFP_NOIO);
          while (!rq) {
                  DEFINE_WAIT(wait);
                  struct request_list *rl = &q->rq;
  
                  prepare_to_wait_exclusive(&rl->wait[rw], &wait,
                                  TASK_UNINTERRUPTIBLE);
  
                  rq = get_request(q, rw_flags, bio, GFP_NOIO);
  
                  if (!rq) {
                          struct io_context *ioc;
  
                          blk_add_trace_generic(q, bio, rw, BLK_TA_SLEEPRQ);
  
                          __generic_unplug_device(q);
                          spin_unlock_irq(q->queue_lock);
                          io_schedule();
  
                          /*
                           * After sleeping, we become a "batching" process and
                           * will be able to allocate at least one request, and
                           * up to a big batch of them for a small period time.
                           * See ioc_batching, ioc_set_batching
                           */
                          ioc = current_io_context(GFP_NOIO, q->node);
                          ioc_set_batching(q, ioc);
  
                          spin_lock_irq(q->queue_lock);
                  }
                  finish_wait(&rl->wait[rw], &wait);
          }
  
          return rq;
  }
  
  struct request *blk_get_request(struct request_queue *q, int rw, gfp_t gfp_mask)
  {
          struct request *rq;
  
          BUG_ON(rw != READ && rw != WRITE);
  
          spin_lock_irq(q->queue_lock);
          if (gfp_mask & __GFP_WAIT) {
                  rq = get_request_wait(q, rw, NULL);
          } else {
                  rq = get_request(q, rw, NULL, gfp_mask);
                  if (!rq)
                          spin_unlock_irq(q->queue_lock);
          }
          /* q->queue_lock is unlocked at this point */
  
          return rq;
  }
  EXPORT_SYMBOL(blk_get_request);
  
  /**
   * blk_start_queueing - initiate dispatch of requests to device
   * @q:          request queue to kick into gear
   *
   * This is basically a helper to remove the need to know whether a queue
   * is plugged or not if someone just wants to initiate dispatch of requests
   * for this queue.
   *
   * The queue lock must be held with interrupts disabled.
   */
  void blk_start_queueing(struct request_queue *q)
  {
          if (!blk_queue_plugged(q))
                  q->request_fn(q);
          else
                  __generic_unplug_device(q);
  }
  EXPORT_SYMBOL(blk_start_queueing);
  
  /**
   * blk_requeue_request - put a request back on queue
   * @q:          request queue where request should be inserted
   * @rq:         request to be inserted
   *
   * Description:
   *    Drivers often keep queueing requests until the hardware cannot accept
   *    more, when that condition happens we need to put the request back
   *    on the queue. Must be called with queue lock held.
   */
  void blk_requeue_request(struct request_queue *q, struct request *rq)
  {
          blk_add_trace_rq(q, rq, BLK_TA_REQUEUE);
  
          if (blk_rq_tagged(rq))
                  blk_queue_end_tag(q, rq);
  
          elv_requeue_request(q, rq);
  }
  
  EXPORT_SYMBOL(blk_requeue_request);
  
  /**
   * blk_insert_request - insert a special request in to a request queue
   * @q:          request queue where request should be inserted
   * @rq:         request to be inserted
   * @at_head:    insert request at head or tail of queue
   * @data:       private data
   *
   * Description:
   *    Many block devices need to execute commands asynchronously, so they don't
   *    block the whole kernel from preemption during request execution.  This is
   *    accomplished normally by inserting aritficial requests tagged as
   *    REQ_SPECIAL in to the corresponding request queue, and letting them be
   *    scheduled for actual execution by the request queue.
   *
   *    We have the option of inserting the head or the tail of the queue.
   *    Typically we use the tail for new ioctls and so forth.  We use the head
   *    of the queue for things like a QUEUE_FULL message from a device, or a
   *    host that is unable to accept a particular command.
   */
  void blk_insert_request(struct request_queue *q, struct request *rq,
                          int at_head, void *data)
  {
          int where = at_head ? ELEVATOR_INSERT_FRONT : ELEVATOR_INSERT_BACK;
          unsigned long flags;
  
          /*
           * tell I/O scheduler that this isn't a regular read/write (ie it
           * must not attempt merges on this) and that it acts as a soft
           * barrier
           */
          rq->cmd_type = REQ_TYPE_SPECIAL;
          rq->cmd_flags |= REQ_SOFTBARRIER;
  
          rq->special = data;
  
          spin_lock_irqsave(q->queue_lock, flags);
  
          /*
           * If command is tagged, release the tag
           */
          if (blk_rq_tagged(rq))
                  blk_queue_end_tag(q, rq);
  
          drive_stat_acct(rq, 1);
          __elv_add_request(q, rq, where, 0);
          blk_start_queueing(q);
          spin_unlock_irqrestore(q->queue_lock, flags);
  }
  
  EXPORT_SYMBOL(blk_insert_request);
  
  static int __blk_rq_unmap_user(struct bio *bio)
  {
          int ret = 0;
  
          if (bio) {
                  if (bio_flagged(bio, BIO_USER_MAPPED))
                          bio_unmap_user(bio);
                  else
                          ret = bio_uncopy_user(bio);
          }
  
          return ret;
  }
  
  int blk_rq_append_bio(struct request_queue *q, struct request *rq,
                        struct bio *bio)
  {
          if (!rq->bio)
                  blk_rq_bio_prep(q, rq, bio);
          else if (!ll_back_merge_fn(q, rq, bio))
                  return -EINVAL;
          else {
                  rq->biotail->bi_next = bio;
                  rq->biotail = bio;
  
                  rq->data_len += bio->bi_size;
          }
          return 0;
  }
  EXPORT_SYMBOL(blk_rq_append_bio);
  
  static int __blk_rq_map_user(struct request_queue *q, struct request *rq,
                               void __user *ubuf, unsigned int len)
  {
          unsigned long uaddr;
          struct bio *bio, *orig_bio;
          int reading, ret;
  
          reading = rq_data_dir(rq) == READ;
  
          /*
           * if alignment requirement is satisfied, map in user pages for
           * direct dma. else, set up kernel bounce buffers
           */
          uaddr = (unsigned long) ubuf;
          if (!(uaddr & queue_dma_alignment(q)) && !(len & queue_dma_alignment(q)))
                  bio = bio_map_user(q, NULL, uaddr, len, reading);
          else
                  bio = bio_copy_user(q, uaddr, len, reading);
  
          if (IS_ERR(bio))
                  return PTR_ERR(bio);
  
          orig_bio = bio;
          blk_queue_bounce(q, &bio);
  
          /*
           * We link the bounce buffer in and could have to traverse it
           * later so we have to get a ref to prevent it from being freed
           */
          bio_get(bio);
  
          ret = blk_rq_append_bio(q, rq, bio);
          if (!ret)
                  return bio->bi_size;
  
          /* if it was boucned we must call the end io function */
          bio_endio(bio, 0);
          __blk_rq_unmap_user(orig_bio);
          bio_put(bio);
          return ret;
  }
  
  /**
   * blk_rq_map_user - map user data to a request, for REQ_BLOCK_PC usage
   * @q:          request queue where request should be inserted
   * @rq:         request structure to fill
   * @ubuf:       the user buffer
   * @len:        length of user data
   *
   * Description:
   *    Data will be mapped directly for zero copy io, if possible. Otherwise
   *    a kernel bounce buffer is used.
   *
   *    A matching blk_rq_unmap_user() must be issued at the end of io, while
   *    still in process context.
   *
   *    Note: The mapped bio may need to be bounced through blk_queue_bounce()
   *    before being submitted to the device, as pages mapped may be out of
   *    reach. It's the callers responsibility to make sure this happens. The
   *    original bio must be passed back in to blk_rq_unmap_user() for proper
   *    unmapping.
   */
  int blk_rq_map_user(struct request_queue *q, struct request *rq,
                      void __user *ubuf, unsigned long len)
  {
          unsigned long bytes_read = 0;
          struct bio *bio = NULL;
          int ret;
  
          if (len > (q->max_hw_sectors << 9))
                  return -EINVAL;
          if (!len || !ubuf)
                  return -EINVAL;
  
          while (bytes_read != len) {
                  unsigned long map_len, end, start;
  
                  map_len = min_t(unsigned long, len - bytes_read, BIO_MAX_SIZE);
                  end = ((unsigned long)ubuf + map_len + PAGE_SIZE - 1)
                                                                  >> PAGE_SHIFT;
                  start = (unsigned long)ubuf >> PAGE_SHIFT;
  
                  /*
                   * A bad offset could cause us to require BIO_MAX_PAGES + 1
                   * pages. If this happens we just lower the requested
                   * mapping len by a page so that we can fit
                   */
                  if (end - start > BIO_MAX_PAGES)
                          map_len -= PAGE_SIZE;
  
                  ret = __blk_rq_map_user(q, rq, ubuf, map_len);
                  if (ret < 0)
                          goto unmap_rq;
                  if (!bio)
                          bio = rq->bio;
                  bytes_read += ret;
                  ubuf += ret;
          }
  
          rq->buffer = rq->data = NULL;
          return 0;
  unmap_rq:
          blk_rq_unmap_user(bio);
          return ret;
  }
  
  EXPORT_SYMBOL(blk_rq_map_user);
  
  /**
   * blk_rq_map_user_iov - map user data to a request, for REQ_BLOCK_PC usage
   * @q:          request queue where request should be inserted
   * @rq:         request to map data to
   * @iov:        pointer to the iovec
   * @iov_count:  number of elements in the iovec
   * @len:        I/O byte count
   *
   * Description:
   *    Data will be mapped directly for zero copy io, if possible. Otherwise
   *    a kernel bounce buffer is used.
   *
   *    A matching blk_rq_unmap_user() must be issued at the end of io, while
   *    still in process context.
   *
   *    Note: The mapped bio may need to be bounced through blk_queue_bounce()
   *    before being submitted to the device, as pages mapped may be out of
   *    reach. It's the callers responsibility to make sure this happens. The
   *    original bio must be passed back in to blk_rq_unmap_user() for proper
   *    unmapping.
   */
  int blk_rq_map_user_iov(struct request_queue *q, struct request *rq,
                          struct sg_iovec *iov, int iov_count, unsigned int len)
  {
          struct bio *bio;
  
          if (!iov || iov_count <= 0)
                  return -EINVAL;
  
          /* we don't allow misaligned data like bio_map_user() does.  If the
           * user is using sg, they're expected to know the alignment constraints
           * and respect them accordingly */
          bio = bio_map_user_iov(q, NULL, iov, iov_count, rq_data_dir(rq)== READ);
          if (IS_ERR(bio))
                  return PTR_ERR(bio);
  
          if (bio->bi_size != len) {
                  bio_endio(bio, 0);
                  bio_unmap_user(bio);
                  return -EINVAL;
          }
  
          bio_get(bio);
          blk_rq_bio_prep(q, rq, bio);
          rq->buffer = rq->data = NULL;
          return 0;
  }
  
  EXPORT_SYMBOL(blk_rq_map_user_iov);
  
  /**
   * blk_rq_unmap_user - unmap a request with user data
   * @bio:               start of bio list
   *
   * Description:
   *    Unmap a rq previously mapped by blk_rq_map_user(). The caller must
   *    supply the original rq->bio from the blk_rq_map_user() return, since
   *    the io completion may have changed rq->bio.
   */
  int blk_rq_unmap_user(struct bio *bio)
  {
          struct bio *mapped_bio;
          int ret = 0, ret2;
  
          while (bio) {
                  mapped_bio = bio;
                  if (unlikely(bio_flagged(bio, BIO_BOUNCED)))
                          mapped_bio = bio->bi_private;
  
                  ret2 = __blk_rq_unmap_user(mapped_bio);
                  if (ret2 && !ret)
                          ret = ret2;
  
                  mapped_bio = bio;
                  bio = bio->bi_next;
                  bio_put(mapped_bio);
          }
  
          return ret;
  }
  
  EXPORT_SYMBOL(blk_rq_unmap_user);
  
  /**
   * blk_rq_map_kern - map kernel data to a request, for REQ_BLOCK_PC usage
   * @q:          request queue where request should be inserted
   * @rq:         request to fill
   * @kbuf:       the kernel buffer
   * @len:        length of user data
   * @gfp_mask:   memory allocation flags
   */
  int blk_rq_map_kern(struct request_queue *q, struct request *rq, void *kbuf,
                      unsigned int len, gfp_t gfp_mask)
  {
          struct bio *bio;
  
          if (len > (q->max_hw_sectors << 9))
                  return -EINVAL;
          if (!len || !kbuf)
                  return -EINVAL;
  
          bio = bio_map_kern(q, kbuf, len, gfp_mask);
          if (IS_ERR(bio))
                  return PTR_ERR(bio);
  
          if (rq_data_dir(rq) == WRITE)
                  bio->bi_rw |= (1 << BIO_RW);
  
          blk_rq_bio_prep(q, rq, bio);
          blk_queue_bounce(q, &rq->bio);
          rq->buffer = rq->data = NULL;
          return 0;
  }
  
  EXPORT_SYMBOL(blk_rq_map_kern);
  
  /**
   * blk_execute_rq_nowait - insert a request into queue for execution
   * @q:          queue to insert the request in
   * @bd_disk:    matching gendisk
   * @rq:         request to insert
   * @at_head:    insert request at head or tail of queue
   * @done:       I/O completion handler
   *
   * Description:
   *    Insert a fully prepared request at the back of the io scheduler queue
   *    for execution.  Don't wait for completion.
   */
  void blk_execute_rq_nowait(struct request_queue *q, struct gendisk *bd_disk,
                             struct request *rq, int at_head,
                             rq_end_io_fn *done)
  {
          int where = at_head ? ELEVATOR_INSERT_FRONT : ELEVATOR_INSERT_BACK;
  
          rq->rq_disk = bd_disk;
          rq->cmd_flags |= REQ_NOMERGE;
          rq->end_io = done;
          WARN_ON(irqs_disabled());
          spin_lock_irq(q->queue_lock);
          __elv_add_request(q, rq, where, 1);
          __generic_unplug_device(q);
          spin_unlock_irq(q->queue_lock);
  }
  EXPORT_SYMBOL_GPL(blk_execute_rq_nowait);
  
  /**
   * blk_execute_rq - insert a request into queue for execution
   * @q:          queue to insert the request in
   * @bd_disk:    matching gendisk
   * @rq:         request to insert
   * @at_head:    insert request at head or tail of queue
   *
   * Description:
   *    Insert a fully prepared request at the back of the io scheduler queue
   *    for execution and wait for completion.
   */
  int blk_execute_rq(struct request_queue *q, struct gendisk *bd_disk,
                     struct request *rq, int at_head)
  {
          DECLARE_COMPLETION_ONSTACK(wait);
          char sense[SCSI_SENSE_BUFFERSIZE];
          int err = 0;
  
          /*
           * we need an extra reference to the request, so we can look at
           * it after io completion
           */
          rq->ref_count++;
  
          if (!rq->sense) {
                  memset(sense, 0, sizeof(sense));
                  rq->sense = sense;
                  rq->sense_len = 0;
          }
  
          rq->end_io_data = &wait;
          blk_execute_rq_nowait(q, bd_disk, rq, at_head, blk_end_sync_rq);
          wait_for_completion(&wait);
  
          if (rq->errors)
                  err = -EIO;
  
          return err;
  }
  
  EXPORT_SYMBOL(blk_execute_rq);
  
  static void bio_end_empty_barrier(struct bio *bio, int err)
  {
          if (err)
                  clear_bit(BIO_UPTODATE, &bio->bi_flags);
  
          complete(bio->bi_private);
  }
  
  /**
   * blkdev_issue_flush - queue a flush
   * @bdev:       blockdev to issue flush for
   * @error_sector:       error sector
   *
   * Description:
   *    Issue a flush for the block device in question. Caller can supply
   *    room for storing the error offset in case of a flush error, if they
   *    wish to.  Caller must run wait_for_completion() on its own.
   */
  int blkdev_issue_flush(struct block_device *bdev, sector_t *error_sector)
  {
          DECLARE_COMPLETION_ONSTACK(wait);
          struct request_queue *q;
          struct bio *bio;
          int ret;
  
          if (bdev->bd_disk == NULL)
                  return -ENXIO;
  
          q = bdev_get_queue(bdev);
          if (!q)
                  return -ENXIO;
  
          bio = bio_alloc(GFP_KERNEL, 0);
          if (!bio)
                  return -ENOMEM;
  
          bio->bi_end_io = bio_end_empty_barrier;
          bio->bi_private = &wait;
          bio->bi_bdev = bdev;
          submit_bio(1 << BIO_RW_BARRIER, bio);
  
          wait_for_completion(&wait);
  
          /*
           * The driver must store the error location in ->bi_sector, if
           * it supports it. For non-stacked drivers, this should be copied
           * from rq->sector.
           */
          if (error_sector)
                  *error_sector = bio->bi_sector;
  
          ret = 0;
          if (!bio_flagged(bio, BIO_UPTODATE))
                  ret = -EIO;
  
          bio_put(bio);
          return ret;
  }
  
  EXPORT_SYMBOL(blkdev_issue_flush);
  
  static void drive_stat_acct(struct request *rq, int new_io)
  {
          int rw = rq_data_dir(rq);
  
          if (!blk_fs_request(rq) || !rq->rq_disk)
                  return;
  
          if (!new_io) {
                  __disk_stat_inc(rq->rq_disk, merges[rw]);
          } else {
                  disk_round_stats(rq->rq_disk);
                  rq->rq_disk->in_flight++;
          }
  }
  
  /*
   * add-request adds a request to the linked list.
   * queue lock is held and interrupts disabled, as we muck with the
   * request queue list.
   */
  static inline void add_request(struct request_queue * q, struct request * req)
  {
          drive_stat_acct(req, 1);
  
          /*
           * elevator indicated where it wants this request to be
           * inserted at elevator_merge time
           */
          __elv_add_request(q, req, ELEVATOR_INSERT_SORT, 0);
  }
   
  /*
   * disk_round_stats()   - Round off the performance stats on a struct
   * disk_stats.
   *
   * The average IO queue length and utilisation statistics are maintained
   * by observing the current state of the queue length and the amount of
   * time it has been in this state for.
   *
   * Normally, that accounting is done on IO completion, but that can result
   * in more than a second's worth of IO being accounted for within any one
   * second, leading to >100% utilisation.  To deal with that, we call this
   * function to do a round-off before returning the results when reading
   * /proc/diskstats.  This accounts immediately for all queue usage up to
   * the current jiffies and restarts the counters again.
   */
  void disk_round_stats(struct gendisk *disk)
  {
          unsigned long now = jiffies;
  
          if (now == disk->stamp)
                  return;
  
          if (disk->in_flight) {
                  __disk_stat_add(disk, time_in_queue,
                                  disk->in_flight * (now - disk->stamp));
                  __disk_stat_add(disk, io_ticks, (now - disk->stamp));
          }
          disk->stamp = now;
  }
  
  EXPORT_SYMBOL_GPL(disk_round_stats);
  
  /*
   * queue lock must be held
   */
  void __blk_put_request(struct request_queue *q, struct request *req)
  {
          if (unlikely(!q))
                  return;
          if (unlikely(--req->ref_count))
                  return;
  
          elv_completed_request(q, req);
  
          /*
           * Request may not have originated from ll_rw_blk. if not,
           * it didn't come out of our reserved rq pools
           */
          if (req->cmd_flags & REQ_ALLOCED) {
                  int rw = rq_data_dir(req);
                  int priv = req->cmd_flags & REQ_ELVPRIV;
  
                  BUG_ON(!list_empty(&req->queuelist));
                  BUG_ON(!hlist_unhashed(&req->hash));
  
                  blk_free_request(q, req);
                  freed_request(q, rw, priv);
          }
  }
  
  EXPORT_SYMBOL_GPL(__blk_put_request);
  
  void blk_put_request(struct request *req)
  {
          unsigned long flags;
          struct request_queue *q = req->q;
  
          /*
           * Gee, IDE calls in w/ NULL q.  Fix IDE and remove the
           * following if (q) test.
           */
          if (q) {
                  spin_lock_irqsave(q->queue_lock, flags);
                  __blk_put_request(q, req);
                  spin_unlock_irqrestore(q->queue_lock, flags);
          }
  }
  
  EXPORT_SYMBOL(blk_put_request);
  
  /**
   * blk_end_sync_rq - executes a completion event on a request
   * @rq: request to complete
   * @error: end io status of the request
   */
  void blk_end_sync_rq(struct request *rq, int error)
  {
          struct completion *waiting = rq->end_io_data;
  
          rq->end_io_data = NULL;
          __blk_put_request(rq->q, rq);
  
          /*
           * complete last, if this is a stack request the process (and thus
           * the rq pointer) could be invalid right after this complete()
           */
          complete(waiting);
  }
  EXPORT_SYMBOL(blk_end_sync_rq);
  
  /*
   * Has to be called with the request spinlock acquired
   */
  static int attempt_merge(struct request_queue *q, struct request *req,
                            struct request *next)
  {
          if (!rq_mergeable(req) || !rq_mergeable(next))
                  return 0;
  
          /*
           * not contiguous
           */
          if (req->sector + req->nr_sectors != next->sector)
                  return 0;
  
          if (rq_data_dir(req) != rq_data_dir(next)
              || req->rq_disk != next->rq_disk
              || next->special)
                  return 0;
  
          /*
           * If we are allowed to merge, then append bio list
           * from next to rq and release next. merge_requests_fn
           * will have updated segment counts, update sector
           * counts here.
           */
          if (!ll_merge_requests_fn(q, req, next))
                  return 0;
  
          /*
           * At this point we have either done a back merge
           * or front merge. We need the smaller start_time of
           * the merged requests to be the current request
           * for accounting purposes.
           */
          if (time_after(req->start_time, next->start_time))
                  req->start_time = next->start_time;
  
          req->biotail->bi_next = next->bio;
          req->biotail = next->biotail;
  
          req->nr_sectors = req->hard_nr_sectors += next->hard_nr_sectors;
  
          elv_merge_requests(q, req, next);
  
          if (req->rq_disk) {
                  disk_round_stats(req->rq_disk);
                  req->rq_disk->in_flight--;
          }
  
          req->ioprio = ioprio_best(req->ioprio, next->ioprio);
  
          __blk_put_request(q, next);
          return 1;
  }
  
  static inline int attempt_back_merge(struct request_queue *q,
                                       struct request *rq)
  {
          struct request *next = elv_latter_request(q, rq);
  
          if (next)
                  return attempt_merge(q, rq, next);
  
          return 0;
  }
  
  static inline int attempt_front_merge(struct request_queue *q,
                                        struct request *rq)
  {
          struct request *prev = elv_former_request(q, rq);
  
          if (prev)
                  return attempt_merge(q, prev, rq);
  
          return 0;
  }
  
  static void init_request_from_bio(struct request *req, struct bio *bio)
  {
          req->cmd_type = REQ_TYPE_FS;
  
          /*
           * inherit FAILFAST from bio (for read-ahead, and explicit FAILFAST)
           */
          if (bio_rw_ahead(bio) || bio_failfast(bio))
                  req->cmd_flags |= REQ_FAILFAST;
  
          /*
           * REQ_BARRIER implies no merging, but lets make it explicit
           */
          if (unlikely(bio_barrier(bio)))
                  req->cmd_flags |= (REQ_HARDBARRIER | REQ_NOMERGE);
  
          if (bio_sync(bio))
                  req->cmd_flags |= REQ_RW_SYNC;
          if (bio_rw_meta(bio))
                  req->cmd_flags |= REQ_RW_META;
  
          req->errors = 0;
          req->hard_sector = req->sector = bio->bi_sector;
          req->ioprio = bio_prio(bio);
          req->start_time = jiffies;
          blk_rq_bio_prep(req->q, req, bio);
  }
  
  static int __make_request(struct request_queue *q, struct bio *bio)
  {
          struct request *req;
          int el_ret, nr_sectors, barrier, err;
          const unsigned short prio = bio_prio(bio);
          const int sync = bio_sync(bio);
          int rw_flags;
  
          nr_sectors = bio_sectors(bio);
  
          /*
           * low level driver can indicate that it wants pages above a
           * certain limit bounced to low memory (ie for highmem, or even
           * ISA dma in theory)
           */
          blk_queue_bounce(q, &bio);
  
          barrier = bio_barrier(bio);
          if (unlikely(barrier) && (q->next_ordered == QUEUE_ORDERED_NONE)) {
                  err = -EOPNOTSUPP;
                  goto end_io;
          }
  
          spin_lock_irq(q->queue_lock);
  
          if (unlikely(barrier) || elv_queue_empty(q))
                  goto get_rq;
  
          el_ret = elv_merge(q, &req, bio);
          switch (el_ret) {
                  case ELEVATOR_BACK_MERGE:
                          BUG_ON(!rq_mergeable(req));
  
                          if (!ll_back_merge_fn(q, req, bio))
                                  break;
  
                          blk_add_trace_bio(q, bio, BLK_TA_BACKMERGE);
  
                          req->biotail->bi_next = bio;
                          req->biotail = bio;
                          req->nr_sectors = req->hard_nr_sectors += nr_sectors;
                          req->ioprio = ioprio_best(req->ioprio, prio);
                          drive_stat_acct(req, 0);
                          if (!attempt_back_merge(q, req))
                                  elv_merged_request(q, req, el_ret);
                          goto out;
  
                  case ELEVATOR_FRONT_MERGE:
                          BUG_ON(!rq_mergeable(req));
  
                          if (!ll_front_merge_fn(q, req, bio))
                                  break;
  
                          blk_add_trace_bio(q, bio, BLK_TA_FRONTMERGE);
  
                          bio->bi_next = req->bio;
                          req->bio = bio;
  
                          /*
                           * may not be valid. if the low level driver said
                           * it didn't need a bounce buffer then it better
                           * not touch req->buffer either...
                           */
                          req->buffer = bio_data(bio);
                          req->current_nr_sectors = bio_cur_sectors(bio);
                          req->hard_cur_sectors = req->current_nr_sectors;
                          req->sector = req->hard_sector = bio->bi_sector;
                          req->nr_sectors = req->hard_nr_sectors += nr_sectors;
                          req->ioprio = ioprio_best(req->ioprio, prio);
                          drive_stat_acct(req, 0);
                          if (!attempt_front_merge(q, req))
                                  elv_merged_request(q, req, el_ret);
                          goto out;
  
                  /* ELV_NO_MERGE: elevator says don't/can't merge. */
                  default:
                          ;
          }
  
  get_rq:
          /*
           * This sync check and mask will be re-done in init_request_from_bio(),
           * but we need to set it earlier to expose the sync flag to the
           * rq allocator and io schedulers.
           */
          rw_flags = bio_data_dir(bio);
          if (sync)
                  rw_flags |= REQ_RW_SYNC;
  
          /*
           * Grab a free request. This is might sleep but can not fail.
           * Returns with the queue unlocked.
           */
          req = get_request_wait(q, rw_flags, bio);
  
          /*
           * After dropping the lock and possibly sleeping here, our request
           * may now be mergeable after it had proven unmergeable (above).
           * We don't worry about that case for efficiency. It won't happen
           * often, and the elevators are able to handle it.
           */
          init_request_from_bio(req, bio);
  
          spin_lock_irq(q->queue_lock);
          if (elv_queue_empty(q))
                  blk_plug_device(q);
          add_request(q, req);
  out:
          if (sync)
                  __generic_unplug_device(q);
  
          spin_unlock_irq(q->queue_lock);
          return 0;
  
  end_io:
          bio_endio(bio, err);
          return 0;
  }
  
  /*
   * If bio->bi_dev is a partition, remap the location
   */
  static inline void blk_partition_remap(struct bio *bio)
  {
          struct block_device *bdev = bio->bi_bdev;
  
          if (bio_sectors(bio) && bdev != bdev->bd_contains) {
                  struct hd_struct *p = bdev->bd_part;
                  const int rw = bio_data_dir(bio);
  
                  p->sectors[rw] += bio_sectors(bio);
                  p->ios[rw]++;
  
                  bio->bi_sector += p->start_sect;
                  bio->bi_bdev = bdev->bd_contains;
  
                  blk_add_trace_remap(bdev_get_queue(bio->bi_bdev), bio,
                                      bdev->bd_dev, bio->bi_sector,
                                      bio->bi_sector - p->start_sect);
          }
  }
  
  static void handle_bad_sector(struct bio *bio)
  {
          char b[BDEVNAME_SIZE];
  
          printk(KERN_INFO "attempt to access beyond end of device\n");
          printk(KERN_INFO "%s: rw=%ld, want=%Lu, limit=%Lu\n",
                          bdevname(bio->bi_bdev, b),
                          bio->bi_rw,
                          (unsigned long long)bio->bi_sector + bio_sectors(bio),
                          (long long)(bio->bi_bdev->bd_inode->i_size >> 9));
  
          set_bit(BIO_EOF, &bio->bi_flags);
  }
  
  #ifdef CONFIG_FAIL_MAKE_REQUEST
  
  static DECLARE_FAULT_ATTR(fail_make_request);
  
  static int __init setup_fail_make_request(char *str)
  {
          return setup_fault_attr(&fail_make_request, str);
  }
  __setup("fail_make_request=", setup_fail_make_request);
  
  static int should_fail_request(struct bio *bio)
  {
          if ((bio->bi_bdev->bd_disk->flags & GENHD_FL_FAIL) ||
              (bio->bi_bdev->bd_part && bio->bi_bdev->bd_part->make_it_fail))
                  return should_fail(&fail_make_request, bio->bi_size);
  
          return 0;
  }
  
  static int __init fail_make_request_debugfs(void)
  {
          return init_fault_attr_dentries(&fail_make_request,
                                          "fail_make_request");
  }
  
  late_initcall(fail_make_request_debugfs);
  
  #else /* CONFIG_FAIL_MAKE_REQUEST */
  
  static inline int should_fail_request(struct bio *bio)
  {
          return 0;
  }
  
  #endif /* CONFIG_FAIL_MAKE_REQUEST */
  
  /*
   * Check whether this bio extends beyond the end of the device.
   */
  static inline int bio_check_eod(struct bio *bio, unsigned int nr_sectors)
  {
          sector_t maxsector;
  
          if (!nr_sectors)
                  return 0;
  
          /* Test device or partition size, when known. */
          maxsector = bio->bi_bdev->bd_inode->i_size >> 9;
          if (maxsector) {
                  sector_t sector = bio->bi_sector;
  
                  if (maxsector < nr_sectors || maxsector - nr_sectors < sector) {
                          /*
                           * This may well happen - the kernel calls bread()
                           * without checking the size of the device, e.g., when
                           * mounting a device.
                           */
                          handle_bad_sector(bio);
                          return 1;
                  }
          }
  
          return 0;
  }
  
  /**
   * generic_make_request: hand a buffer to its device driver for I/O
   * @bio:  The bio describing the location in memory and on the device.
   *
   * generic_make_request() is used to make I/O requests of block
   * devices. It is passed a &struct bio, which describes the I/O that needs
   * to be done.
   *
   * generic_make_request() does not return any status.  The
   * success/failure status of the request, along with notification of
   * completion, is delivered asynchronously through the bio->bi_end_io
   * function described (one day) else where.
   *
   * The caller of generic_make_request must make sure that bi_io_vec
   * are set to describe the memory buffer, and that bi_dev and bi_sector are
   * set to describe the device address, and the
   * bi_end_io and optionally bi_private are set to describe how
   * completion notification should be signaled.
   *
   * generic_make_request and the drivers it calls may use bi_next if this
   * bio happens to be merged with someone else, and may change bi_dev and
   * bi_sector for remaps as it sees fit.  So the values of these fields
   * should NOT be depended on after the call to generic_make_request.
   */
  static inline void __generic_make_request(struct bio *bio)
  {
          struct request_queue *q;
          sector_t old_sector;
          int ret, nr_sectors = bio_sectors(bio);
          dev_t old_dev;
          int err = -EIO;
  
          might_sleep();
  
          if (bio_check_eod(bio, nr_sectors))
                  goto end_io;
  
          /*
           * Resolve the mapping until finished. (drivers are
           * still free to implement/resolve their own stacking
           * by explicitly returning 0)
           *
           * NOTE: we don't repeat the blk_size check for each new device.
           * Stacking drivers are expected to know what they are doing.
           */
          old_sector = -1;
          old_dev = 0;
          do {
                  char b[BDEVNAME_SIZE];
  
                  q = bdev_get_queue(bio->bi_bdev);
                  if (!q) {
                          printk(KERN_ERR
                                 "generic_make_request: Trying to access "
                                  "nonexistent block-device %s (%Lu)\n",
                                  bdevname(bio->bi_bdev, b),
                                  (long long) bio->bi_sector);
  end_io:
                          bio_endio(bio, err);
                          break;
                  }
  
                  if (unlikely(nr_sectors > q->max_hw_sectors)) {
                          printk("bio too big device %s (%u > %u)\n", 
                                  bdevname(bio->bi_bdev, b),
                                  bio_sectors(bio),
                                  q->max_hw_sectors);
                          goto end_io;
                  }
  
                  if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
                          goto end_io;
  
                  if (should_fail_request(bio))
                          goto end_io;
  
                  /*
                   * If this device has partitions, remap block n
                   * of partition p to block n+start(p) of the disk.
                   */
                  blk_partition_remap(bio);
  
                  if (old_sector != -1)
                          blk_add_trace_remap(q, bio, old_dev, bio->bi_sector,
                                              old_sector);
  
                  blk_add_trace_bio(q, bio, BLK_TA_QUEUE);
  
                  old_sector = bio->bi_sector;
                  old_dev = bio->bi_bdev->bd_dev;
  
                  if (bio_check_eod(bio, nr_sectors))
                          goto end_io;
                  if (bio_empty_barrier(bio) && !q->prepare_flush_fn) {
                          err = -EOPNOTSUPP;
                          goto end_io;
                  }
  
                  ret = q->make_request_fn(q, bio);
          } while (ret);
  }
  
  /*
   * We only want one ->make_request_fn to be active at a time,
   * else stack usage with stacked devices could be a problem.
   * So use current->bio_{list,tail} to keep a list of requests
   * submited by a make_request_fn function.
   * current->bio_tail is also used as a flag to say if
   * generic_make_request is currently active in this task or not.
   * If it is NULL, then no make_request is active.  If it is non-NULL,
   * then a make_request is active, and new requests should be added
   * at the tail
   */
  void generic_make_request(struct bio *bio)
  {
          if (current->bio_tail) {
                  /* make_request is active */
                  *(current->bio_tail) = bio;
                  bio->bi_next = NULL;
                  current->bio_tail = &bio->bi_next;
                  return;
          }
          /* following loop may be a bit non-obvious, and so deserves some
           * explanation.
           * Before entering the loop, bio->bi_next is NULL (as all callers
           * ensure that) so we have a list with a single bio.
           * We pretend that we have just taken it off a longer list, so
           * we assign bio_list to the next (which is NULL) and bio_tail
           * to &bio_list, thus initialising the bio_list of new bios to be
           * added.  __generic_make_request may indeed add some more bios
           * through a recursive call to generic_make_request.  If it
           * did, we find a non-NULL value in bio_list and re-enter the loop
           * from the top.  In this case we really did just take the bio
           * of the top of the list (no pretending) and so fixup bio_list and
           * bio_tail or bi_next, and call into __generic_make_request again.
           *
           * The loop was structured like this to make only one call to
           * __generic_make_request (which is important as it is large and
           * inlined) and to keep the structure simple.
           */
          BUG_ON(bio->bi_next);
          do {
                  current->bio_list = bio->bi_next;
                  if (bio->bi_next == NULL)
                          current->bio_tail = &current->bio_list;
                  else
                          bio->bi_next = NULL;
                  __generic_make_request(bio);
                  bio = current->bio_list;
          } while (bio);
          current->bio_tail = NULL; /* deactivate */
  }
  
  EXPORT_SYMBOL(generic_make_request);
  
  /**
   * submit_bio: submit a bio to the block device layer for I/O
   * @rw: whether to %READ or %WRITE, or maybe to %READA (read ahead)
   * @bio: The &struct bio which describes the I/O
   *
   * submit_bio() is very similar in purpose to generic_make_request(), and
   * uses that function to do most of the work. Both are fairly rough
   * interfaces, @bio must be presetup and ready for I/O.
   *
   */
  void submit_bio(int rw, struct bio *bio)
  {
          int count = bio_sectors(bio);
  
          bio->bi_rw |= rw;
  
          /*
           * If it's a regular read/write or a barrier with data attached,
           * go through the normal accounting stuff before submission.
           */
          if (!bio_empty_barrier(bio)) {
  
                  BIO_BUG_ON(!bio->bi_size);
                  BIO_BUG_ON(!bio->bi_io_vec);
  
                  if (rw & WRITE) {
                          count_vm_events(PGPGOUT, count);
                  } else {
                          task_io_account_read(bio->bi_size);
                          count_vm_events(PGPGIN, count);
                  }
  
                  if (unlikely(block_dump)) {
                          char b[BDEVNAME_SIZE];
                          printk(KERN_DEBUG "%s(%d): %s block %Lu on %s\n",
                          current->comm, task_pid_nr(current),
                                  (rw & WRITE) ? "WRITE" : "READ",
                                  (unsigned long long)bio->bi_sector,
                                  bdevname(bio->bi_bdev,b));
                  }
          }
  
          generic_make_request(bio);
  }
  
  EXPORT_SYMBOL(submit_bio);
  
  static void blk_recalc_rq_sectors(struct request *rq, int nsect)
  {
          if (blk_fs_request(rq)) {
                  rq->hard_sector += nsect;
                  rq->hard_nr_sectors -= nsect;
  
                  /*
                   * Move the I/O submission pointers ahead if required.
                   */
                  if ((rq->nr_sectors >= rq->hard_nr_sectors) &&
                      (rq->sector <= rq->hard_sector)) {
                          rq->sector = rq->hard_sector;
                          rq->nr_sectors = rq->hard_nr_sectors;
                          rq->hard_cur_sectors = bio_cur_sectors(rq->bio);
                          rq->current_nr_sectors = rq->hard_cur_sectors;
                          rq->buffer = bio_data(rq->bio);
                  }
  
                  /*
                   * if total number of sectors is less than the first segment
                   * size, something has gone terribly wrong
                   */
                  if (rq->nr_sectors < rq->current_nr_sectors) {
                          printk("blk: request botched\n");
                          rq->nr_sectors = rq->current_nr_sectors;
                  }
          }
  }
  
  static int __end_that_request_first(struct request *req, int uptodate,
                                      int nr_bytes)
  {
          int total_bytes, bio_nbytes, error, next_idx = 0;
          struct bio *bio;
  
          blk_add_trace_rq(req->q, req, BLK_TA_COMPLETE);
  
          /*
           * extend uptodate bool to allow < 0 value to be direct io error
           */
          error = 0;
          if (end_io_error(uptodate))
                  error = !uptodate ? -EIO : uptodate;
  
          /*
           * for a REQ_BLOCK_PC request, we want to carry any eventual
           * sense key with us all the way through
           */
          if (!blk_pc_request(req))
                  req->errors = 0;
  
          if (!uptodate) {
                  if (blk_fs_request(req) && !(req->cmd_flags & REQ_QUIET))
                          printk("end_request: I/O error, dev %s, sector %llu\n",
                                  req->rq_disk ? req->rq_disk->disk_name : "?",
                                  (unsigned long long)req->sector);
          }
  
          if (blk_fs_request(req) && req->rq_disk) {
                  const int rw = rq_data_dir(req);
  
                  disk_stat_add(req->rq_disk, sectors[rw], nr_bytes >> 9);
          }
  
          total_bytes = bio_nbytes = 0;
          while ((bio = req->bio) != NULL) {
                  int nbytes;
  
                  /*
                   * For an empty barrier request, the low level driver must
                   * store a potential error location in ->sector. We pass
                   * that back up in ->bi_sector.
                   */
                  if (blk_empty_barrier(req))
                          bio->bi_sector = req->sector;
  
                  if (nr_bytes >= bio->bi_size) {
                          req->bio = bio->bi_next;
                          nbytes = bio->bi_size;
                          req_bio_endio(req, bio, nbytes, error);
                          next_idx = 0;
                          bio_nbytes = 0;
                  } else {
                          int idx = bio->bi_idx + next_idx;
  
                          if (unlikely(bio->bi_idx >= bio->bi_vcnt)) {
                                  blk_dump_rq_flags(req, "__end_that");
                                  printk("%s: bio idx %d >= vcnt %d\n",
                                                  __FUNCTION__,
                                                  bio->bi_idx, bio->bi_vcnt);
                                  break;
                          }
  
                          nbytes = bio_iovec_idx(bio, idx)->bv_len;
                          BIO_BUG_ON(nbytes > bio->bi_size);
  
                          /*
                           * not a complete bvec done
                           */
                          if (unlikely(nbytes > nr_bytes)) {
                                  bio_nbytes += nr_bytes;
                                  total_bytes += nr_bytes;
                                  break;
                          }
  
                          /*
                           * advance to the next vector
                           */
                          next_idx++;
                          bio_nbytes += nbytes;
                  }
  
                  total_bytes += nbytes;
                  nr_bytes -= nbytes;
  
                  if ((bio = req->bio)) {
                          /*
                           * end more in this run, or just return 'not-done'
                           */
                          if (unlikely(nr_bytes <= 0))
                                  break;
                  }
          }
  
          /*
           * completely done
           */
          if (!req->bio)
                  return 0;
  
          /*
           * if the request wasn't completed, update state
           */
          if (bio_nbytes) {
                  req_bio_endio(req, bio, bio_nbytes, error);
                  bio->bi_idx += next_idx;
                  bio_iovec(bio)->bv_offset += nr_bytes;
                  bio_iovec(bio)->bv_len -= nr_bytes;
          }
  
          blk_recalc_rq_sectors(req, total_bytes >> 9);
          blk_recalc_rq_segments(req);
          return 1;
  }
  
  /**
   * end_that_request_first - end I/O on a request
   * @req:      the request being processed
   * @uptodate: 1 for success, 0 for I/O error, < 0 for specific error
   * @nr_sectors: number of sectors to end I/O on
   *
   * Description:
   *     Ends I/O on a number of sectors attached to @req, and sets it up
   *     for the next range of segments (if any) in the cluster.
   *
   * Return:
   *     0 - we are done with this request, call end_that_request_last()
   *     1 - still buffers pending for this request
   **/
  int end_that_request_first(struct request *req, int uptodate, int nr_sectors)
  {
          return __end_that_request_first(req, uptodate, nr_sectors << 9);
  }
  
  EXPORT_SYMBOL(end_that_request_first);
  
  /**
   * end_that_request_chunk - end I/O on a request
   * @req:      the request being processed
   * @uptodate: 1 for success, 0 for I/O error, < 0 for specific error
   * @nr_bytes: number of bytes to complete
   *
   * Description:
   *     Ends I/O on a number of bytes attached to @req, and sets it up
   *     for the next range of segments (if any). Like end_that_request_first(),
   *     but deals with bytes instead of sectors.
   *
   * Return:
   *     0 - we are done with this request, call end_that_request_last()
   *     1 - still buffers pending for this request
   **/
  int end_that_request_chunk(struct request *req, int uptodate, int nr_bytes)
  {
          return __end_that_request_first(req, uptodate, nr_bytes);
  }
  
  EXPORT_SYMBOL(end_that_request_chunk);
  
  /*
   * splice the completion data to a local structure and hand off to
   * process_completion_queue() to complete the requests
   */
  static void blk_done_softirq(struct softirq_action *h)
  {
          struct list_head *cpu_list, local_list;
  
          local_irq_disable();
          cpu_list = &__get_cpu_var(blk_cpu_done);
          list_replace_init(cpu_list, &local_list);
          local_irq_enable();
  
          while (!list_empty(&local_list)) {
                  struct request *rq = list_entry(local_list.next, struct request, donelist);
  
                  list_del_init(&rq->donelist);
                  rq->q->softirq_done_fn(rq);
          }
  }
  
  static int __cpuinit blk_cpu_notify(struct notifier_block *self, unsigned long action,
                            void *hcpu)
  {
          /*
           * If a CPU goes away, splice its entries to the current CPU
           * and trigger a run of the softirq
           */
          if (action == CPU_DEAD || action == CPU_DEAD_FROZEN) {
                  int cpu = (unsigned long) hcpu;
  
                  local_irq_disable();
                  list_splice_init(&per_cpu(blk_cpu_done, cpu),
                                   &__get_cpu_var(blk_cpu_done));
                  raise_softirq_irqoff(BLOCK_SOFTIRQ);
                  local_irq_enable();
          }
  
          return NOTIFY_OK;
  }
  
  
  static struct notifier_block blk_cpu_notifier __cpuinitdata = {
          .notifier_call  = blk_cpu_notify,
  };
  
  /**
   * blk_complete_request - end I/O on a request
   * @req:      the request being processed
   *
   * Description:
   *     Ends all I/O on a request. It does not handle partial completions,
   *     unless the driver actually implements this in its completion callback
   *     through requeueing. The actual completion happens out-of-order,
   *     through a softirq handler. The user must have registered a completion
   *     callback through blk_queue_softirq_done().
   **/
  
  void blk_complete_request(struct request *req)
  {
          struct list_head *cpu_list;
          unsigned long flags;
  
          BUG_ON(!req->q->softirq_done_fn);
                  
          local_irq_save(flags);
  
          cpu_list = &__get_cpu_var(blk_cpu_done);
          list_add_tail(&req->donelist, cpu_list);
          raise_softirq_irqoff(BLOCK_SOFTIRQ);
  
          local_irq_restore(flags);
  }
  
  EXPORT_SYMBOL(blk_complete_request);
          
  /*
   * queue lock must be held
   */
  void end_that_request_last(struct request *req, int uptodate)
  {
          struct gendisk *disk = req->rq_disk;
          int error;
  
          /*
           * extend uptodate bool to allow < 0 value to be direct io error
           */
          error = 0;
          if (end_io_error(uptodate))
                  error = !uptodate ? -EIO : uptodate;
  
          if (unlikely(laptop_mode) && blk_fs_request(req))
                  laptop_io_completion();
  
          /*
           * Account IO completion.  bar_rq isn't accounted as a normal
           * IO on queueing nor completion.  Accounting the containing
           * request is enough.
           */
          if (disk && blk_fs_request(req) && req != &req->q->bar_rq) {
                  unsigned long duration = jiffies - req->start_time;
                  const int rw = rq_data_dir(req);
  
                  __disk_stat_inc(disk, ios[rw]);
                  __disk_stat_add(disk, ticks[rw], duration);
                  disk_round_stats(disk);
                  disk->in_flight--;
          }
          if (req->end_io)
                  req->end_io(req, error);
          else
                  __blk_put_request(req->q, req);
  }
  
  EXPORT_SYMBOL(end_that_request_last);
  
  static inline void __end_request(struct request *rq, int uptodate,
                                   unsigned int nr_bytes, int dequeue)
  {
          if (!end_that_request_chunk(rq, uptodate, nr_bytes)) {
                  if (dequeue)
                          blkdev_dequeue_request(rq);
                  add_disk_randomness(rq->rq_disk);
                  end_that_request_last(rq, uptodate);
          }
  }
  
  static unsigned int rq_byte_size(struct request *rq)
  {
          if (blk_fs_request(rq))
                  return rq->hard_nr_sectors << 9;
  
          return rq->data_len;
  }
  
  /**
   * end_queued_request - end all I/O on a queued request
   * @rq:         the request being processed
   * @uptodate:   error value or 0/1 uptodate flag
   *
   * Description:
   *     Ends all I/O on a request, and removes it from the block layer queues.
   *     Not suitable for normal IO completion, unless the driver still has
   *     the request attached to the block layer.
   *
   **/
  void end_queued_request(struct request *rq, int uptodate)
  {
          __end_request(rq, uptodate, rq_byte_size(rq), 1);
  }
  EXPORT_SYMBOL(end_queued_request);
  
  /**
   * end_dequeued_request - end all I/O on a dequeued request
   * @rq:         the request being processed
   * @uptodate:   error value or 0/1 uptodate flag
   *
   * Description:
   *     Ends all I/O on a request. The request must already have been
   *     dequeued using blkdev_dequeue_request(), as is normally the case
   *     for most drivers.
   *
   **/
  void end_dequeued_request(struct request *rq, int uptodate)
  {
          __end_request(rq, uptodate, rq_byte_size(rq), 0);
  }
  EXPORT_SYMBOL(end_dequeued_request);
  
  
  /**
   * end_request - end I/O on the current segment of the request
   * @req:        the request being processed
   * @uptodate:   error value or 0/1 uptodate flag
   *
   * Description:
   *     Ends I/O on the current segment of a request. If that is the only
   *     remaining segment, the request is also completed and freed.
   *
   *     This is a remnant of how older block drivers handled IO completions.
   *     Modern drivers typically end IO on the full request in one go, unless
   *     they have a residual value to account for. For that case this function
   *     isn't really useful, unless the residual just happens to be the
   *     full current segment. In other words, don't use this function in new
   *     code. Either use end_request_completely(), or the
   *     end_that_request_chunk() (along with end_that_request_last()) for
   *     partial completions.
   *
   **/
  void end_request(struct request *req, int uptodate)
  {
          __end_request(req, uptodate, req->hard_cur_sectors << 9, 1);
  }
  EXPORT_SYMBOL(end_request);
  
  static void blk_rq_bio_prep(struct request_queue *q, struct request *rq,
                              struct bio *bio)
  {
          /* first two bits are identical in rq->cmd_flags and bio->bi_rw */
          rq->cmd_flags |= (bio->bi_rw & 3);
  
          rq->nr_phys_segments = bio_phys_segments(q, bio);
          rq->nr_hw_segments = bio_hw_segments(q, bio);
          rq->current_nr_sectors = bio_cur_sectors(bio);
          rq->hard_cur_sectors = rq->current_nr_sectors;
          rq->hard_nr_sectors = rq->nr_sectors = bio_sectors(bio);
          rq->buffer = bio_data(bio);
          rq->data_len = bio->bi_size;
  
          rq->bio = rq->biotail = bio;
  
          if (bio->bi_bdev)
                  rq->rq_disk = bio->bi_bdev->bd_disk;
  }
  
  int kblockd_schedule_work(struct work_struct *work)
  {
          return queue_work(kblockd_workqueue, work);
  }
  
  EXPORT_SYMBOL(kblockd_schedule_work);
  
  void kblockd_flush_work(struct work_struct *work)
  {
          cancel_work_sync(work);
  }
  EXPORT_SYMBOL(kblockd_flush_work);
  
  int __init blk_dev_init(void)
  {
          int i;
  
          kblockd_workqueue = create_workqueue("kblockd");
          if (!kblockd_workqueue)
                  panic("Failed to create kblockd\n");
  
          request_cachep = kmem_cache_create("blkdev_requests",
                          sizeof(struct request), 0, SLAB_PANIC, NULL);
  
          requestq_cachep = kmem_cache_create("blkdev_queue",
                          sizeof(struct request_queue), 0, SLAB_PANIC, NULL);
  
          iocontext_cachep = kmem_cache_create("blkdev_ioc",
                          sizeof(struct io_context), 0, SLAB_PANIC, NULL);
  
          for_each_possible_cpu(i)
                  INIT_LIST_HEAD(&per_cpu(blk_cpu_done, i));
  
          open_softirq(BLOCK_SOFTIRQ, blk_done_softirq, NULL);
          register_hotcpu_notifier(&blk_cpu_notifier);
  
          blk_max_low_pfn = max_low_pfn - 1;
          blk_max_pfn = max_pfn - 1;
  
          return 0;
  }
  
  /*
   * IO Context helper functions
   */
  void put_io_context(struct io_context *ioc)
  {
          if (ioc == NULL)
                  return;
  
          BUG_ON(atomic_read(&ioc->refcount) == 0);
  
          if (atomic_dec_and_test(&ioc->refcount)) {
                  struct cfq_io_context *cic;
  
                  rcu_read_lock();
                  if (ioc->aic && ioc->aic->dtor)
                          ioc->aic->dtor(ioc->aic);
                  if (ioc->cic_root.rb_node != NULL) {
                          struct rb_node *n = rb_first(&ioc->cic_root);
  
                          cic = rb_entry(n, struct cfq_io_context, rb_node);
                          cic->dtor(ioc);
                  }
                  rcu_read_unlock();
  
                  kmem_cache_free(iocontext_cachep, ioc);
          }
  }
  EXPORT_SYMBOL(put_io_context);
  
  /* Called by the exitting task */
  void exit_io_context(void)
  {
          struct io_context *ioc;
          struct cfq_io_context *cic;
  
          task_lock(current);
          ioc = current->io_context;
          current->io_context = NULL;
          task_unlock(current);
  
          ioc->task = NULL;
          if (ioc->aic && ioc->aic->exit)
                  ioc->aic->exit(ioc->aic);
          if (ioc->cic_root.rb_node != NULL) {
                  cic = rb_entry(rb_first(&ioc->cic_root), struct cfq_io_context, rb_node);
                  cic->exit(ioc);
          }
  
          put_io_context(ioc);
  }
  
  /*
   * If the current task has no IO context then create one and initialise it.
   * Otherwise, return its existing IO context.
   *
   * This returned IO context doesn't have a specifically elevated refcount,
   * but since the current task itself holds a reference, the context can be
   * used in general code, so long as it stays within `current` context.
   */
  static struct io_context *current_io_context(gfp_t gfp_flags, int node)
  {
          struct task_struct *tsk = current;
          struct io_context *ret;
  
          ret = tsk->io_context;
          if (likely(ret))
                  return ret;
  
          ret = kmem_cache_alloc_node(iocontext_cachep, gfp_flags, node);
          if (ret) {
                  atomic_set(&ret->refcount, 1);
                  ret->task = current;
                  ret->ioprio_changed = 0;
                  ret->last_waited = jiffies; /* doesn't matter... */
                  ret->nr_batch_requests = 0; /* because this is 0 */
                  ret->aic = NULL;
                  ret->cic_root.rb_node = NULL;
                  ret->ioc_data = NULL;
                  /* make sure set_task_ioprio() sees the settings above */
                  smp_wmb();
                  tsk->io_context = ret;
          }
  
          return ret;
  }
  
  /*
   * If the current task has no IO context then create one and initialise it.
   * If it does have a context, take a ref on it.
   *
   * This is always called in the context of the task which submitted the I/O.
   */
  struct io_context *get_io_context(gfp_t gfp_flags, int node)
  {
          struct io_context *ret;
          ret = current_io_context(gfp_flags, node);
          if (likely(ret))
                  atomic_inc(&ret->refcount);
          return ret;
  }
  EXPORT_SYMBOL(get_io_context);
  
  void copy_io_context(struct io_context **pdst, struct io_context **psrc)
  {
          struct io_context *src = *psrc;
          struct io_context *dst = *pdst;
  
          if (src) {
                  BUG_ON(atomic_read(&src->refcount) == 0);
                  atomic_inc(&src->refcount);
                  put_io_context(dst);
                  *pdst = src;
          }
  }
  EXPORT_SYMBOL(copy_io_context);
  
  void swap_io_context(struct io_context **ioc1, struct io_context **ioc2)
  {
          struct io_context *temp;
          temp = *ioc1;
          *ioc1 = *ioc2;
          *ioc2 = temp;
  }
  EXPORT_SYMBOL(swap_io_context);
  
  /*
   * sysfs parts below
   */
  struct queue_sysfs_entry {
          struct attribute attr;
          ssize_t (*show)(struct request_queue *, char *);
          ssize_t (*store)(struct request_queue *, const char *, size_t);
  };
  
  static ssize_t
  queue_var_show(unsigned int var, char *page)
  {
          return sprintf(page, "%d\n", var);
  }
  
  static ssize_t
  queue_var_store(unsigned long *var, const char *page, size_t count)
  {
          char *p = (char *) page;
  
          *var = simple_strtoul(p, &p, 10);
          return count;
  }
  
  static ssize_t queue_requests_show(struct request_queue *q, char *page)
  {
          return queue_var_show(q->nr_requests, (page));
  }
  
  static ssize_t
  queue_requests_store(struct request_queue *q, const char *page, size_t count)
  {
          struct request_list *rl = &q->rq;
          unsigned long nr;
          int ret = queue_var_store(&nr, page, count);
          if (nr < BLKDEV_MIN_RQ)
                  nr = BLKDEV_MIN_RQ;
  
          spin_lock_irq(q->queue_lock);
          q->nr_requests = nr;
          blk_queue_congestion_threshold(q);
  
          if (rl->count[READ] >= queue_congestion_on_threshold(q))
                  blk_set_queue_congested(q, READ);
          else if (rl->count[READ] < queue_congestion_off_threshold(q))
                  blk_clear_queue_congested(q, READ);
  
          if (rl->count[WRITE] >= queue_congestion_on_threshold(q))
                  blk_set_queue_congested(q, WRITE);
          else if (rl->count[WRITE] < queue_congestion_off_threshold(q))
                  blk_clear_queue_congested(q, WRITE);
  
          if (rl->count[READ] >= q->nr_requests) {
                  blk_set_queue_full(q, READ);
          } else if (rl->count[READ]+1 <= q->nr_requests) {
                  blk_clear_queue_full(q, READ);
                  wake_up(&rl->wait[READ]);
          }
  
          if (rl->count[WRITE] >= q->nr_requests) {
                  blk_set_queue_full(q, WRITE);
          } else if (rl->count[WRITE]+1 <= q->nr_requests) {
                  blk_clear_queue_full(q, WRITE);
                  wake_up(&rl->wait[WRITE]);
          }
          spin_unlock_irq(q->queue_lock);
          return ret;
  }
  
  static ssize_t queue_ra_show(struct request_queue *q, char *page)
  {
          int ra_kb = q->backing_dev_info.ra_pages << (PAGE_CACHE_SHIFT - 10);
  
          return queue_var_show(ra_kb, (page));
  }
  
  static ssize_t
  queue_ra_store(struct request_queue *q, const char *page, size_t count)
  {
          unsigned long ra_kb;
          ssize_t ret = queue_var_store(&ra_kb, page, count);
  
          spin_lock_irq(q->queue_lock);
          q->backing_dev_info.ra_pages = ra_kb >> (PAGE_CACHE_SHIFT - 10);
          spin_unlock_irq(q->queue_lock);
  
          return ret;
  }
  
  static ssize_t queue_max_sectors_show(struct request_queue *q, char *page)
  {
          int max_sectors_kb = q->max_sectors >> 1;
  
          return queue_var_show(max_sectors_kb, (page));
  }
  
  static ssize_t
  queue_max_sectors_store(struct request_queue *q, const char *page, size_t count)
  {
          unsigned long max_sectors_kb,
                          max_hw_sectors_kb = q->max_hw_sectors >> 1,
                          page_kb = 1 << (PAGE_CACHE_SHIFT - 10);
          ssize_t ret = queue_var_store(&max_sectors_kb, page, count);
  
          if (max_sectors_kb > max_hw_sectors_kb || max_sectors_kb < page_kb)
                  return -EINVAL;
          /*
           * Take the queue lock to update the readahead and max_sectors
           * values synchronously:
           */
          spin_lock_irq(q->queue_lock);
          q->max_sectors = max_sectors_kb << 1;
          spin_unlock_irq(q->queue_lock);
  
          return ret;
  }
  
  static ssize_t queue_max_hw_sectors_show(struct request_queue *q, char *page)
  {
          int max_hw_sectors_kb = q->max_hw_sectors >> 1;
  
          return queue_var_show(max_hw_sectors_kb, (page));
  }
  
  
  static struct queue_sysfs_entry queue_requests_entry = {
          .attr = {.name = "nr_requests", .mode = S_IRUGO | S_IWUSR },
          .show = queue_requests_show,
          .store = queue_requests_store,
  };
  
  static struct queue_sysfs_entry queue_ra_entry = {
          .attr = {.name = "read_ahead_kb", .mode = S_IRUGO | S_IWUSR },
          .show = queue_ra_show,
          .store = queue_ra_store,
  };
  
  static struct queue_sysfs_entry queue_max_sectors_entry = {
          .attr = {.name = "max_sectors_kb", .mode = S_IRUGO | S_IWUSR },
          .show = queue_max_sectors_show,
          .store = queue_max_sectors_store,
  };
  
  static struct queue_sysfs_entry queue_max_hw_sectors_entry = {
          .attr = {.name = "max_hw_sectors_kb", .mode = S_IRUGO },
          .show = queue_max_hw_sectors_show,
  };
  
  static struct queue_sysfs_entry queue_iosched_entry = {
          .attr = {.name = "scheduler", .mode = S_IRUGO | S_IWUSR },
          .show = elv_iosched_show,
          .store = elv_iosched_store,
  };
  
  static struct attribute *default_attrs[] = {
          &queue_requests_entry.attr,
          &queue_ra_entry.attr,
          &queue_max_hw_sectors_entry.attr,
          &queue_max_sectors_entry.attr,
          &queue_iosched_entry.attr,
          NULL,
  };
  
  #define to_queue(atr) container_of((atr), struct queue_sysfs_entry, attr)
  
  static ssize_t
  queue_attr_show(struct kobject *kobj, struct attribute *attr, char *page)
  {
          struct queue_sysfs_entry *entry = to_queue(attr);
          struct request_queue *q =
                  container_of(kobj, struct request_queue, kobj);
          ssize_t res;
  
          if (!entry->show)
                  return -EIO;
          mutex_lock(&q->sysfs_lock);
          if (test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)) {
                  mutex_unlock(&q->sysfs_lock);
                  return -ENOENT;
          }
          res = entry->show(q, page);
          mutex_unlock(&q->sysfs_lock);
          return res;
  }
  
  static ssize_t
  queue_attr_store(struct kobject *kobj, struct attribute *attr,
                      const char *page, size_t length)
  {
          struct queue_sysfs_entry *entry = to_queue(attr);
          struct request_queue *q = container_of(kobj, struct request_queue, kobj);
  
          ssize_t res;
  
          if (!entry->store)
                  return -EIO;
          mutex_lock(&q->sysfs_lock);
          if (test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)) {
                  mutex_unlock(&q->sysfs_lock);
                  return -ENOENT;
          }
          res = entry->store(q, page, length);
          mutex_unlock(&q->sysfs_lock);
          return res;
  }
  
  static struct sysfs_ops queue_sysfs_ops = {
          .show   = queue_attr_show,
          .store  = queue_attr_store,
  };
  
  static struct kobj_type queue_ktype = {
          .sysfs_ops      = &queue_sysfs_ops,
          .default_attrs  = default_attrs,
          .release        = blk_release_queue,
  };
  
  int blk_register_queue(struct gendisk *disk)
  {
          int ret;
  
          struct request_queue *q = disk->queue;
  
          if (!q || !q->request_fn)
                  return -ENXIO;
  
          ret = kobject_add(&q->kobj, kobject_get(&disk->dev.kobj),
                            "%s", "queue");
          if (ret < 0)
                  return ret;
  
          kobject_uevent(&q->kobj, KOBJ_ADD);
  
          ret = elv_register_queue(q);
          if (ret) {
                  kobject_uevent(&q->kobj, KOBJ_REMOVE);
                  kobject_del(&q->kobj);
                  return ret;
          }
  
          return 0;
  }
  
  void blk_unregister_queue(struct gendisk *disk)
  {
          struct request_queue *q = disk->queue;
  
          if (q && q->request_fn) {
                  elv_unregister_queue(q);
  
                  kobject_uevent(&q->kobj, KOBJ_REMOVE);
                  kobject_del(&q->kobj);
                  kobject_put(&disk->dev.kobj);
          }
  }