FreeBSD/Linux Kernel Cross Reference
sys/fs/buffer.c

     /*
      *  linux/fs/buffer.c
      *
      *  Copyright (C) 1991, 1992, 2002  Linus Torvalds
      */
     
     /*
      * Start bdflush() with kernel_thread not syscall - Paul Gortmaker, 12/95
      *
     * Removed a lot of unnecessary code and simplified things now that
     * the buffer cache isn't our primary cache - Andrew Tridgell 12/96
     *
     * Speed up hash, lru, and free list operations.  Use gfp() for allocating
     * hash table, use SLAB cache for buffer heads. SMP threading.  -DaveM
     *
     * Added 32k buffer block sizes - these are required older ARM systems. - RMK
     *
     * async buffer flushing, 1999 Andrea Arcangeli <andrea@suse.de>
     */
    
    #include <linux/kernel.h>
    #include <linux/syscalls.h>
    #include <linux/fs.h>
    #include <linux/mm.h>
    #include <linux/percpu.h>
    #include <linux/slab.h>
    #include <linux/capability.h>
    #include <linux/blkdev.h>
    #include <linux/file.h>
    #include <linux/quotaops.h>
    #include <linux/highmem.h>
    #include <linux/export.h>
    #include <linux/writeback.h>
    #include <linux/hash.h>
    #include <linux/suspend.h>
    #include <linux/buffer_head.h>
    #include <linux/task_io_accounting_ops.h>
    #include <linux/bio.h>
    #include <linux/notifier.h>
    #include <linux/cpu.h>
    #include <linux/bitops.h>
    #include <linux/mpage.h>
    #include <linux/bit_spinlock.h>
    
    static int fsync_buffers_list(spinlock_t *lock, struct list_head *list);
    
    #define BH_ENTRY(list) list_entry((list), struct buffer_head, b_assoc_buffers)
    
    void init_buffer(struct buffer_head *bh, bh_end_io_t *handler, void *private)
    {
            bh->b_end_io = handler;
            bh->b_private = private;
    }
    EXPORT_SYMBOL(init_buffer);
    
    static int sleep_on_buffer(void *word)
    {
            io_schedule();
            return 0;
    }
    
    void __lock_buffer(struct buffer_head *bh)
    {
            wait_on_bit_lock(&bh->b_state, BH_Lock, sleep_on_buffer,
                                                            TASK_UNINTERRUPTIBLE);
    }
    EXPORT_SYMBOL(__lock_buffer);
    
    void unlock_buffer(struct buffer_head *bh)
    {
            clear_bit_unlock(BH_Lock, &bh->b_state);
            smp_mb__after_clear_bit();
            wake_up_bit(&bh->b_state, BH_Lock);
    }
    EXPORT_SYMBOL(unlock_buffer);
    
    /*
     * Block until a buffer comes unlocked.  This doesn't stop it
     * from becoming locked again - you have to lock it yourself
     * if you want to preserve its state.
     */
    void __wait_on_buffer(struct buffer_head * bh)
    {
            wait_on_bit(&bh->b_state, BH_Lock, sleep_on_buffer, TASK_UNINTERRUPTIBLE);
    }
    EXPORT_SYMBOL(__wait_on_buffer);
    
    static void
    __clear_page_buffers(struct page *page)
    {
            ClearPagePrivate(page);
            set_page_private(page, 0);
            page_cache_release(page);
    }
    
    
    static int quiet_error(struct buffer_head *bh)
    {
            if (!test_bit(BH_Quiet, &bh->b_state) && printk_ratelimit())
                   return 0;
           return 1;
   }
   
   
   static void buffer_io_error(struct buffer_head *bh)
   {
           char b[BDEVNAME_SIZE];
           printk(KERN_ERR "Buffer I/O error on device %s, logical block %Lu\n",
                           bdevname(bh->b_bdev, b),
                           (unsigned long long)bh->b_blocknr);
   }
   
   /*
    * End-of-IO handler helper function which does not touch the bh after
    * unlocking it.
    * Note: unlock_buffer() sort-of does touch the bh after unlocking it, but
    * a race there is benign: unlock_buffer() only use the bh's address for
    * hashing after unlocking the buffer, so it doesn't actually touch the bh
    * itself.
    */
   static void __end_buffer_read_notouch(struct buffer_head *bh, int uptodate)
   {
           if (uptodate) {
                   set_buffer_uptodate(bh);
           } else {
                   /* This happens, due to failed READA attempts. */
                   clear_buffer_uptodate(bh);
           }
           unlock_buffer(bh);
   }
   
   /*
    * Default synchronous end-of-IO handler..  Just mark it up-to-date and
    * unlock the buffer. This is what ll_rw_block uses too.
    */
   void end_buffer_read_sync(struct buffer_head *bh, int uptodate)
   {
           __end_buffer_read_notouch(bh, uptodate);
           put_bh(bh);
   }
   EXPORT_SYMBOL(end_buffer_read_sync);
   
   void end_buffer_write_sync(struct buffer_head *bh, int uptodate)
   {
           char b[BDEVNAME_SIZE];
   
           if (uptodate) {
                   set_buffer_uptodate(bh);
           } else {
                   if (!quiet_error(bh)) {
                           buffer_io_error(bh);
                           printk(KERN_WARNING "lost page write due to "
                                           "I/O error on %s\n",
                                          bdevname(bh->b_bdev, b));
                   }
                   set_buffer_write_io_error(bh);
                   clear_buffer_uptodate(bh);
           }
           unlock_buffer(bh);
           put_bh(bh);
   }
   EXPORT_SYMBOL(end_buffer_write_sync);
   
   /*
    * Various filesystems appear to want __find_get_block to be non-blocking.
    * But it's the page lock which protects the buffers.  To get around this,
    * we get exclusion from try_to_free_buffers with the blockdev mapping's
    * private_lock.
    *
    * Hack idea: for the blockdev mapping, i_bufferlist_lock contention
    * may be quite high.  This code could TryLock the page, and if that
    * succeeds, there is no need to take private_lock. (But if
    * private_lock is contended then so is mapping->tree_lock).
    */
   static struct buffer_head *
   __find_get_block_slow(struct block_device *bdev, sector_t block)
   {
           struct inode *bd_inode = bdev->bd_inode;
           struct address_space *bd_mapping = bd_inode->i_mapping;
           struct buffer_head *ret = NULL;
           pgoff_t index;
           struct buffer_head *bh;
           struct buffer_head *head;
           struct page *page;
           int all_mapped = 1;
   
           index = block >> (PAGE_CACHE_SHIFT - bd_inode->i_blkbits);
           page = find_get_page(bd_mapping, index);
           if (!page)
                   goto out;
   
           spin_lock(&bd_mapping->private_lock);
           if (!page_has_buffers(page))
                   goto out_unlock;
           head = page_buffers(page);
           bh = head;
           do {
                   if (!buffer_mapped(bh))
                           all_mapped = 0;
                   else if (bh->b_blocknr == block) {
                           ret = bh;
                           get_bh(bh);
                           goto out_unlock;
                   }
                   bh = bh->b_this_page;
           } while (bh != head);
   
           /* we might be here because some of the buffers on this page are
            * not mapped.  This is due to various races between
            * file io on the block device and getblk.  It gets dealt with
            * elsewhere, don't buffer_error if we had some unmapped buffers
            */
           if (all_mapped) {
                   char b[BDEVNAME_SIZE];
   
                   printk("__find_get_block_slow() failed. "
                           "block=%llu, b_blocknr=%llu\n",
                           (unsigned long long)block,
                           (unsigned long long)bh->b_blocknr);
                   printk("b_state=0x%08lx, b_size=%zu\n",
                           bh->b_state, bh->b_size);
                   printk("device %s blocksize: %d\n", bdevname(bdev, b),
                           1 << bd_inode->i_blkbits);
           }
   out_unlock:
           spin_unlock(&bd_mapping->private_lock);
           page_cache_release(page);
   out:
           return ret;
   }
   
   /*
    * Kick the writeback threads then try to free up some ZONE_NORMAL memory.
    */
   static void free_more_memory(void)
   {
           struct zone *zone;
           int nid;
   
           wakeup_flusher_threads(1024, WB_REASON_FREE_MORE_MEM);
           yield();
   
           for_each_online_node(nid) {
                   (void)first_zones_zonelist(node_zonelist(nid, GFP_NOFS),
                                                   gfp_zone(GFP_NOFS), NULL,
                                                   &zone);
                   if (zone)
                           try_to_free_pages(node_zonelist(nid, GFP_NOFS), 0,
                                                   GFP_NOFS, NULL);
           }
   }
   
   /*
    * I/O completion handler for block_read_full_page() - pages
    * which come unlocked at the end of I/O.
    */
   static void end_buffer_async_read(struct buffer_head *bh, int uptodate)
   {
           unsigned long flags;
           struct buffer_head *first;
           struct buffer_head *tmp;
           struct page *page;
           int page_uptodate = 1;
   
           BUG_ON(!buffer_async_read(bh));
   
           page = bh->b_page;
           if (uptodate) {
                   set_buffer_uptodate(bh);
           } else {
                   clear_buffer_uptodate(bh);
                   if (!quiet_error(bh))
                           buffer_io_error(bh);
                   SetPageError(page);
           }
   
           /*
            * Be _very_ careful from here on. Bad things can happen if
            * two buffer heads end IO at almost the same time and both
            * decide that the page is now completely done.
            */
           first = page_buffers(page);
           local_irq_save(flags);
           bit_spin_lock(BH_Uptodate_Lock, &first->b_state);
           clear_buffer_async_read(bh);
           unlock_buffer(bh);
           tmp = bh;
           do {
                   if (!buffer_uptodate(tmp))
                           page_uptodate = 0;
                   if (buffer_async_read(tmp)) {
                           BUG_ON(!buffer_locked(tmp));
                           goto still_busy;
                   }
                   tmp = tmp->b_this_page;
           } while (tmp != bh);
           bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
           local_irq_restore(flags);
   
           /*
            * If none of the buffers had errors and they are all
            * uptodate then we can set the page uptodate.
            */
           if (page_uptodate && !PageError(page))
                   SetPageUptodate(page);
           unlock_page(page);
           return;
   
   still_busy:
           bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
           local_irq_restore(flags);
           return;
   }
   
   /*
    * Completion handler for block_write_full_page() - pages which are unlocked
    * during I/O, and which have PageWriteback cleared upon I/O completion.
    */
   void end_buffer_async_write(struct buffer_head *bh, int uptodate)
   {
           char b[BDEVNAME_SIZE];
           unsigned long flags;
           struct buffer_head *first;
           struct buffer_head *tmp;
           struct page *page;
   
           BUG_ON(!buffer_async_write(bh));
   
           page = bh->b_page;
           if (uptodate) {
                   set_buffer_uptodate(bh);
           } else {
                   if (!quiet_error(bh)) {
                           buffer_io_error(bh);
                           printk(KERN_WARNING "lost page write due to "
                                           "I/O error on %s\n",
                                  bdevname(bh->b_bdev, b));
                   }
                   set_bit(AS_EIO, &page->mapping->flags);
                   set_buffer_write_io_error(bh);
                   clear_buffer_uptodate(bh);
                   SetPageError(page);
           }
   
           first = page_buffers(page);
           local_irq_save(flags);
           bit_spin_lock(BH_Uptodate_Lock, &first->b_state);
   
           clear_buffer_async_write(bh);
           unlock_buffer(bh);
           tmp = bh->b_this_page;
           while (tmp != bh) {
                   if (buffer_async_write(tmp)) {
                           BUG_ON(!buffer_locked(tmp));
                           goto still_busy;
                   }
                   tmp = tmp->b_this_page;
           }
           bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
           local_irq_restore(flags);
           end_page_writeback(page);
           return;
   
   still_busy:
           bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
           local_irq_restore(flags);
           return;
   }
   EXPORT_SYMBOL(end_buffer_async_write);
   
   /*
    * If a page's buffers are under async readin (end_buffer_async_read
    * completion) then there is a possibility that another thread of
    * control could lock one of the buffers after it has completed
    * but while some of the other buffers have not completed.  This
    * locked buffer would confuse end_buffer_async_read() into not unlocking
    * the page.  So the absence of BH_Async_Read tells end_buffer_async_read()
    * that this buffer is not under async I/O.
    *
    * The page comes unlocked when it has no locked buffer_async buffers
    * left.
    *
    * PageLocked prevents anyone starting new async I/O reads any of
    * the buffers.
    *
    * PageWriteback is used to prevent simultaneous writeout of the same
    * page.
    *
    * PageLocked prevents anyone from starting writeback of a page which is
    * under read I/O (PageWriteback is only ever set against a locked page).
    */
   static void mark_buffer_async_read(struct buffer_head *bh)
   {
           bh->b_end_io = end_buffer_async_read;
           set_buffer_async_read(bh);
   }
   
   static void mark_buffer_async_write_endio(struct buffer_head *bh,
                                             bh_end_io_t *handler)
   {
           bh->b_end_io = handler;
           set_buffer_async_write(bh);
   }
   
   void mark_buffer_async_write(struct buffer_head *bh)
   {
           mark_buffer_async_write_endio(bh, end_buffer_async_write);
   }
   EXPORT_SYMBOL(mark_buffer_async_write);
   
   
   /*
    * fs/buffer.c contains helper functions for buffer-backed address space's
    * fsync functions.  A common requirement for buffer-based filesystems is
    * that certain data from the backing blockdev needs to be written out for
    * a successful fsync().  For example, ext2 indirect blocks need to be
    * written back and waited upon before fsync() returns.
    *
    * The functions mark_buffer_inode_dirty(), fsync_inode_buffers(),
    * inode_has_buffers() and invalidate_inode_buffers() are provided for the
    * management of a list of dependent buffers at ->i_mapping->private_list.
    *
    * Locking is a little subtle: try_to_free_buffers() will remove buffers
    * from their controlling inode's queue when they are being freed.  But
    * try_to_free_buffers() will be operating against the *blockdev* mapping
    * at the time, not against the S_ISREG file which depends on those buffers.
    * So the locking for private_list is via the private_lock in the address_space
    * which backs the buffers.  Which is different from the address_space 
    * against which the buffers are listed.  So for a particular address_space,
    * mapping->private_lock does *not* protect mapping->private_list!  In fact,
    * mapping->private_list will always be protected by the backing blockdev's
    * ->private_lock.
    *
    * Which introduces a requirement: all buffers on an address_space's
    * ->private_list must be from the same address_space: the blockdev's.
    *
    * address_spaces which do not place buffers at ->private_list via these
    * utility functions are free to use private_lock and private_list for
    * whatever they want.  The only requirement is that list_empty(private_list)
    * be true at clear_inode() time.
    *
    * FIXME: clear_inode should not call invalidate_inode_buffers().  The
    * filesystems should do that.  invalidate_inode_buffers() should just go
    * BUG_ON(!list_empty).
    *
    * FIXME: mark_buffer_dirty_inode() is a data-plane operation.  It should
    * take an address_space, not an inode.  And it should be called
    * mark_buffer_dirty_fsync() to clearly define why those buffers are being
    * queued up.
    *
    * FIXME: mark_buffer_dirty_inode() doesn't need to add the buffer to the
    * list if it is already on a list.  Because if the buffer is on a list,
    * it *must* already be on the right one.  If not, the filesystem is being
    * silly.  This will save a ton of locking.  But first we have to ensure
    * that buffers are taken *off* the old inode's list when they are freed
    * (presumably in truncate).  That requires careful auditing of all
    * filesystems (do it inside bforget()).  It could also be done by bringing
    * b_inode back.
    */
   
   /*
    * The buffer's backing address_space's private_lock must be held
    */
   static void __remove_assoc_queue(struct buffer_head *bh)
   {
           list_del_init(&bh->b_assoc_buffers);
           WARN_ON(!bh->b_assoc_map);
           if (buffer_write_io_error(bh))
                   set_bit(AS_EIO, &bh->b_assoc_map->flags);
           bh->b_assoc_map = NULL;
   }
   
   int inode_has_buffers(struct inode *inode)
   {
           return !list_empty(&inode->i_data.private_list);
   }
   
   /*
    * osync is designed to support O_SYNC io.  It waits synchronously for
    * all already-submitted IO to complete, but does not queue any new
    * writes to the disk.
    *
    * To do O_SYNC writes, just queue the buffer writes with ll_rw_block as
    * you dirty the buffers, and then use osync_inode_buffers to wait for
    * completion.  Any other dirty buffers which are not yet queued for
    * write will not be flushed to disk by the osync.
    */
   static int osync_buffers_list(spinlock_t *lock, struct list_head *list)
   {
           struct buffer_head *bh;
           struct list_head *p;
           int err = 0;
   
           spin_lock(lock);
   repeat:
           list_for_each_prev(p, list) {
                   bh = BH_ENTRY(p);
                   if (buffer_locked(bh)) {
                           get_bh(bh);
                           spin_unlock(lock);
                           wait_on_buffer(bh);
                           if (!buffer_uptodate(bh))
                                   err = -EIO;
                           brelse(bh);
                           spin_lock(lock);
                           goto repeat;
                   }
           }
           spin_unlock(lock);
           return err;
   }
   
   static void do_thaw_one(struct super_block *sb, void *unused)
   {
           char b[BDEVNAME_SIZE];
           while (sb->s_bdev && !thaw_bdev(sb->s_bdev, sb))
                   printk(KERN_WARNING "Emergency Thaw on %s\n",
                          bdevname(sb->s_bdev, b));
   }
   
   static void do_thaw_all(struct work_struct *work)
   {
           iterate_supers(do_thaw_one, NULL);
           kfree(work);
           printk(KERN_WARNING "Emergency Thaw complete\n");
   }
   
   /**
    * emergency_thaw_all -- forcibly thaw every frozen filesystem
    *
    * Used for emergency unfreeze of all filesystems via SysRq
    */
   void emergency_thaw_all(void)
   {
           struct work_struct *work;
   
           work = kmalloc(sizeof(*work), GFP_ATOMIC);
           if (work) {
                   INIT_WORK(work, do_thaw_all);
                   schedule_work(work);
           }
   }
   
   /**
    * sync_mapping_buffers - write out & wait upon a mapping's "associated" buffers
    * @mapping: the mapping which wants those buffers written
    *
    * Starts I/O against the buffers at mapping->private_list, and waits upon
    * that I/O.
    *
    * Basically, this is a convenience function for fsync().
    * @mapping is a file or directory which needs those buffers to be written for
    * a successful fsync().
    */
   int sync_mapping_buffers(struct address_space *mapping)
   {
           struct address_space *buffer_mapping = mapping->private_data;
   
           if (buffer_mapping == NULL || list_empty(&mapping->private_list))
                   return 0;
   
           return fsync_buffers_list(&buffer_mapping->private_lock,
                                           &mapping->private_list);
   }
   EXPORT_SYMBOL(sync_mapping_buffers);
   
   /*
    * Called when we've recently written block `bblock', and it is known that
    * `bblock' was for a buffer_boundary() buffer.  This means that the block at
    * `bblock + 1' is probably a dirty indirect block.  Hunt it down and, if it's
    * dirty, schedule it for IO.  So that indirects merge nicely with their data.
    */
   void write_boundary_block(struct block_device *bdev,
                           sector_t bblock, unsigned blocksize)
   {
           struct buffer_head *bh = __find_get_block(bdev, bblock + 1, blocksize);
           if (bh) {
                   if (buffer_dirty(bh))
                           ll_rw_block(WRITE, 1, &bh);
                   put_bh(bh);
           }
   }
   
   void mark_buffer_dirty_inode(struct buffer_head *bh, struct inode *inode)
   {
           struct address_space *mapping = inode->i_mapping;
           struct address_space *buffer_mapping = bh->b_page->mapping;
   
           mark_buffer_dirty(bh);
           if (!mapping->private_data) {
                   mapping->private_data = buffer_mapping;
           } else {
                   BUG_ON(mapping->private_data != buffer_mapping);
           }
           if (!bh->b_assoc_map) {
                   spin_lock(&buffer_mapping->private_lock);
                   list_move_tail(&bh->b_assoc_buffers,
                                   &mapping->private_list);
                   bh->b_assoc_map = mapping;
                   spin_unlock(&buffer_mapping->private_lock);
           }
   }
   EXPORT_SYMBOL(mark_buffer_dirty_inode);
   
   /*
    * Mark the page dirty, and set it dirty in the radix tree, and mark the inode
    * dirty.
    *
    * If warn is true, then emit a warning if the page is not uptodate and has
    * not been truncated.
    */
   static void __set_page_dirty(struct page *page,
                   struct address_space *mapping, int warn)
   {
           spin_lock_irq(&mapping->tree_lock);
           if (page->mapping) {    /* Race with truncate? */
                   WARN_ON_ONCE(warn && !PageUptodate(page));
                   account_page_dirtied(page, mapping);
                   radix_tree_tag_set(&mapping->page_tree,
                                   page_index(page), PAGECACHE_TAG_DIRTY);
           }
           spin_unlock_irq(&mapping->tree_lock);
           __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
   }
   
   /*
    * Add a page to the dirty page list.
    *
    * It is a sad fact of life that this function is called from several places
    * deeply under spinlocking.  It may not sleep.
    *
    * If the page has buffers, the uptodate buffers are set dirty, to preserve
    * dirty-state coherency between the page and the buffers.  It the page does
    * not have buffers then when they are later attached they will all be set
    * dirty.
    *
    * The buffers are dirtied before the page is dirtied.  There's a small race
    * window in which a writepage caller may see the page cleanness but not the
    * buffer dirtiness.  That's fine.  If this code were to set the page dirty
    * before the buffers, a concurrent writepage caller could clear the page dirty
    * bit, see a bunch of clean buffers and we'd end up with dirty buffers/clean
    * page on the dirty page list.
    *
    * We use private_lock to lock against try_to_free_buffers while using the
    * page's buffer list.  Also use this to protect against clean buffers being
    * added to the page after it was set dirty.
    *
    * FIXME: may need to call ->reservepage here as well.  That's rather up to the
    * address_space though.
    */
   int __set_page_dirty_buffers(struct page *page)
   {
           int newly_dirty;
           struct address_space *mapping = page_mapping(page);
   
           if (unlikely(!mapping))
                   return !TestSetPageDirty(page);
   
           spin_lock(&mapping->private_lock);
           if (page_has_buffers(page)) {
                   struct buffer_head *head = page_buffers(page);
                   struct buffer_head *bh = head;
   
                   do {
                           set_buffer_dirty(bh);
                           bh = bh->b_this_page;
                   } while (bh != head);
           }
           newly_dirty = !TestSetPageDirty(page);
           spin_unlock(&mapping->private_lock);
   
           if (newly_dirty)
                   __set_page_dirty(page, mapping, 1);
           return newly_dirty;
   }
   EXPORT_SYMBOL(__set_page_dirty_buffers);
   
   /*
    * Write out and wait upon a list of buffers.
    *
    * We have conflicting pressures: we want to make sure that all
    * initially dirty buffers get waited on, but that any subsequently
    * dirtied buffers don't.  After all, we don't want fsync to last
    * forever if somebody is actively writing to the file.
    *
    * Do this in two main stages: first we copy dirty buffers to a
    * temporary inode list, queueing the writes as we go.  Then we clean
    * up, waiting for those writes to complete.
    * 
    * During this second stage, any subsequent updates to the file may end
    * up refiling the buffer on the original inode's dirty list again, so
    * there is a chance we will end up with a buffer queued for write but
    * not yet completed on that list.  So, as a final cleanup we go through
    * the osync code to catch these locked, dirty buffers without requeuing
    * any newly dirty buffers for write.
    */
   static int fsync_buffers_list(spinlock_t *lock, struct list_head *list)
   {
           struct buffer_head *bh;
           struct list_head tmp;
           struct address_space *mapping;
           int err = 0, err2;
           struct blk_plug plug;
   
           INIT_LIST_HEAD(&tmp);
           blk_start_plug(&plug);
   
           spin_lock(lock);
           while (!list_empty(list)) {
                   bh = BH_ENTRY(list->next);
                   mapping = bh->b_assoc_map;
                   __remove_assoc_queue(bh);
                   /* Avoid race with mark_buffer_dirty_inode() which does
                    * a lockless check and we rely on seeing the dirty bit */
                   smp_mb();
                   if (buffer_dirty(bh) || buffer_locked(bh)) {
                           list_add(&bh->b_assoc_buffers, &tmp);
                           bh->b_assoc_map = mapping;
                           if (buffer_dirty(bh)) {
                                   get_bh(bh);
                                   spin_unlock(lock);
                                   /*
                                    * Ensure any pending I/O completes so that
                                    * write_dirty_buffer() actually writes the
                                    * current contents - it is a noop if I/O is
                                    * still in flight on potentially older
                                    * contents.
                                    */
                                   write_dirty_buffer(bh, WRITE_SYNC);
   
                                   /*
                                    * Kick off IO for the previous mapping. Note
                                    * that we will not run the very last mapping,
                                    * wait_on_buffer() will do that for us
                                    * through sync_buffer().
                                    */
                                   brelse(bh);
                                   spin_lock(lock);
                           }
                   }
           }
   
           spin_unlock(lock);
           blk_finish_plug(&plug);
           spin_lock(lock);
   
           while (!list_empty(&tmp)) {
                   bh = BH_ENTRY(tmp.prev);
                   get_bh(bh);
                   mapping = bh->b_assoc_map;
                   __remove_assoc_queue(bh);
                   /* Avoid race with mark_buffer_dirty_inode() which does
                    * a lockless check and we rely on seeing the dirty bit */
                   smp_mb();
                   if (buffer_dirty(bh)) {
                           list_add(&bh->b_assoc_buffers,
                                    &mapping->private_list);
                           bh->b_assoc_map = mapping;
                   }
                   spin_unlock(lock);
                   wait_on_buffer(bh);
                   if (!buffer_uptodate(bh))
                           err = -EIO;
                   brelse(bh);
                   spin_lock(lock);
           }
           
           spin_unlock(lock);
           err2 = osync_buffers_list(lock, list);
           if (err)
                   return err;
           else
                   return err2;
   }
   
   /*
    * Invalidate any and all dirty buffers on a given inode.  We are
    * probably unmounting the fs, but that doesn't mean we have already
    * done a sync().  Just drop the buffers from the inode list.
    *
    * NOTE: we take the inode's blockdev's mapping's private_lock.  Which
    * assumes that all the buffers are against the blockdev.  Not true
    * for reiserfs.
    */
   void invalidate_inode_buffers(struct inode *inode)
   {
           if (inode_has_buffers(inode)) {
                   struct address_space *mapping = &inode->i_data;
                   struct list_head *list = &mapping->private_list;
                   struct address_space *buffer_mapping = mapping->private_data;
   
                   spin_lock(&buffer_mapping->private_lock);
                   while (!list_empty(list))
                           __remove_assoc_queue(BH_ENTRY(list->next));
                   spin_unlock(&buffer_mapping->private_lock);
           }
   }
   EXPORT_SYMBOL(invalidate_inode_buffers);
   
   /*
    * Remove any clean buffers from the inode's buffer list.  This is called
    * when we're trying to free the inode itself.  Those buffers can pin it.
    *
    * Returns true if all buffers were removed.
    */
   int remove_inode_buffers(struct inode *inode)
   {
           int ret = 1;
   
           if (inode_has_buffers(inode)) {
                   struct address_space *mapping = &inode->i_data;
                   struct list_head *list = &mapping->private_list;
                   struct address_space *buffer_mapping = mapping->private_data;
   
                   spin_lock(&buffer_mapping->private_lock);
                   while (!list_empty(list)) {
                           struct buffer_head *bh = BH_ENTRY(list->next);
                           if (buffer_dirty(bh)) {
                                   ret = 0;
                                   break;
                           }
                           __remove_assoc_queue(bh);
                   }
                   spin_unlock(&buffer_mapping->private_lock);
           }
           return ret;
   }
   
   /*
    * Create the appropriate buffers when given a page for data area and
    * the size of each buffer.. Use the bh->b_this_page linked list to
    * follow the buffers created.  Return NULL if unable to create more
    * buffers.
    *
    * The retry flag is used to differentiate async IO (paging, swapping)
    * which may not fail from ordinary buffer allocations.
    */
   struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
                   int retry)
   {
           struct buffer_head *bh, *head;
           long offset;
   
   try_again:
           head = NULL;
           offset = PAGE_SIZE;
           while ((offset -= size) >= 0) {
                   bh = alloc_buffer_head(GFP_NOFS);
                   if (!bh)
                           goto no_grow;
   
                   bh->b_this_page = head;
                   bh->b_blocknr = -1;
                   head = bh;
   
                   bh->b_size = size;
   
                   /* Link the buffer to its page */
                   set_bh_page(bh, page, offset);
   
                   init_buffer(bh, NULL, NULL);
           }
           return head;
   /*
    * In case anything failed, we just free everything we got.
    */
   no_grow:
           if (head) {
                   do {
                           bh = head;
                           head = head->b_this_page;
                           free_buffer_head(bh);
                   } while (head);
           }
   
           /*
            * Return failure for non-async IO requests.  Async IO requests
            * are not allowed to fail, so we have to wait until buffer heads
            * become available.  But we don't want tasks sleeping with 
            * partially complete buffers, so all were released above.
            */
           if (!retry)
                   return NULL;
   
           /* We're _really_ low on memory. Now we just
            * wait for old buffer heads to become free due to
            * finishing IO.  Since this is an async request and
            * the reserve list is empty, we're sure there are 
            * async buffer heads in use.
            */
           free_more_memory();
           goto try_again;
   }
   EXPORT_SYMBOL_GPL(alloc_page_buffers);
   
   static inline void
   link_dev_buffers(struct page *page, struct buffer_head *head)
   {
           struct buffer_head *bh, *tail;
   
           bh = head;
           do {
                   tail = bh;
                   bh = bh->b_this_page;
           } while (bh);
           tail->b_this_page = head;
           attach_page_buffers(page, head);
   }
   
   static sector_t blkdev_max_block(struct block_device *bdev, unsigned int size)
   {
           sector_t retval = ~((sector_t)0);
           loff_t sz = i_size_read(bdev->bd_inode);
   
           if (sz) {
                   unsigned int sizebits = blksize_bits(size);
                   retval = (sz >> sizebits);
           }
           return retval;
   }
   
   /*
    * Initialise the state of a blockdev page's buffers.
    */ 
   static sector_t
   init_page_buffers(struct page *page, struct block_device *bdev,
                           sector_t block, int size)
   {
           struct buffer_head *head = page_buffers(page);
           struct buffer_head *bh = head;
           int uptodate = PageUptodate(page);
           sector_t end_block = blkdev_max_block(I_BDEV(bdev->bd_inode), size);
   
           do {
                   if (!buffer_mapped(bh)) {
                           init_buffer(bh, NULL, NULL);
                           bh->b_bdev = bdev;
                           bh->b_blocknr = block;
                           if (uptodate)
                                   set_buffer_uptodate(bh);
                           if (block < end_block)
                                   set_buffer_mapped(bh);
                   }
                   block++;
                   bh = bh->b_this_page;
           } while (bh != head);
   
           /*
            * Caller needs to validate requested block against end of device.
            */
           return end_block;
   }
   
   /*
    * Create the page-cache page that contains the requested block.
    *
    * This is used purely for blockdev mappings.
    */
   static int
   grow_dev_page(struct block_device *bdev, sector_t block,
                   pgoff_t index, int size, int sizebits)
   {
           struct inode *inode = bdev->bd_inode;
           struct page *page;
           struct buffer_head *bh;
           sector_t end_block;
           int ret = 0;            /* Will call free_more_memory() */
   
           page = find_or_create_page(inode->i_mapping, index,
                   (mapping_gfp_mask(inode->i_mapping) & ~__GFP_FS)|__GFP_MOVABLE);
           if (!page)
                   return ret;
   
           BUG_ON(!PageLocked(page));
   
           if (page_has_buffers(page)) {
                   bh = page_buffers(page);
                   if (bh->b_size == size) {
                           end_block = init_page_buffers(page, bdev,
                                                   index << sizebits, size);
                           goto done;
                   }
                   if (!try_to_free_buffers(page))
                           goto failed;
           }
   
           /*
            * Allocate some buffers for this page
            */
           bh = alloc_page_buffers(page, size, 0);
           if (!bh)
                   goto failed;
   
           /*
            * Link the page to the buffers and initialise them.  Take the
            * lock to be atomic wrt __find_get_block(), which does not
            * run under the page lock.
            */
           spin_lock(&inode->i_mapping->private_lock);
          link_dev_buffers(page, bh);
          end_block = init_page_buffers(page, bdev, index << sizebits, size);
          spin_unlock(&inode->i_mapping->private_lock);
  done:
          ret = (block < end_block) ? 1 : -ENXIO;
  failed:
          unlock_page(page);
          page_cache_release(page);
          return ret;
  }
  
  /*
   * Create buffers for the specified block device block's page.  If
   * that page was dirty, the buffers are set dirty also.
   */
  static int
  grow_buffers(struct block_device *bdev, sector_t block, int size)
  {
          pgoff_t index;
          int sizebits;
  
          sizebits = -1;
          do {
                  sizebits++;
          } while ((size << sizebits) < PAGE_SIZE);
  
          index = block >> sizebits;
  
          /*
           * Check for a block which wants to lie outside our maximum possible
           * pagecache index.  (this comparison is done using sector_t types).
           */
          if (unlikely(index != block >> sizebits)) {
                  char b[BDEVNAME_SIZE];
  
                  printk(KERN_ERR "%s: requested out-of-range block %llu for "
                          "device %s\n",
                          __func__, (unsigned long long)block,
                          bdevname(bdev, b));
                  return -EIO;
          }
  
          /* Create a page with the proper size buffers.. */
          return grow_dev_page(bdev, block, index, size, sizebits);
  }
  
  static struct buffer_head *
  __getblk_slow(struct block_device *bdev, sector_t block, int size)
  {
          /* Size must be multiple of hard sectorsize */
          if (unlikely(size & (bdev_logical_block_size(bdev)-1) ||
                          (size < 512 || size > PAGE_SIZE))) {
                  printk(KERN_ERR "getblk(): invalid block size %d requested\n",
                                          size);
                  printk(KERN_ERR "logical block size: %d\n",
                                          bdev_logical_block_size(bdev));
  
                  dump_stack();
                  return NULL;
          }
  
          for (;;) {
                  struct buffer_head *bh;
                  int ret;
  
                  bh = __find_get_block(bdev, block, size);
                  if (bh)
                          return bh;
  
                  ret = grow_buffers(bdev, block, size);
                  if (ret < 0)
                          return NULL;
                  if (ret == 0)
                          free_more_memory();
          }
  }
  
  /*
   * The relationship between dirty buffers and dirty pages:
   *
   * Whenever a page has any dirty buffers, the page's dirty bit is set, and
   * the page is tagged dirty in its radix tree.
   *
   * At all times, the dirtiness of the buffers represents the dirtiness of
   * subsections of the page.  If the page has buffers, the page dirty bit is
   * merely a hint about the true dirty state.
   *
   * When a page is set dirty in its entirety, all its buffers are marked dirty
   * (if the page has buffers).
   *
   * When a buffer is marked dirty, its page is dirtied, but the page's other
   * buffers are not.
   *
   * Also.  When blockdev buffers are explicitly read with bread(), they
   * individually become uptodate.  But their backing page remains not
   * uptodate - even if all of its buffers are uptodate.  A subsequent
   * block_read_full_page() against that page will discover all the uptodate
   * buffers, will set the page uptodate and will perform no I/O.
   */
  
  /**
   * mark_buffer_dirty - mark a buffer_head as needing writeout
   * @bh: the buffer_head to mark dirty
   *
   * mark_buffer_dirty() will set the dirty bit against the buffer, then set its
   * backing page dirty, then tag the page as dirty in its address_space's radix
   * tree and then attach the address_space's inode to its superblock's dirty
   * inode list.
   *
   * mark_buffer_dirty() is atomic.  It takes bh->b_page->mapping->private_lock,
   * mapping->tree_lock and mapping->host->i_lock.
   */
  void mark_buffer_dirty(struct buffer_head *bh)
  {
          WARN_ON_ONCE(!buffer_uptodate(bh));
  
          /*
           * Very *carefully* optimize the it-is-already-dirty case.
           *
           * Don't let the final "is it dirty" escape to before we
           * perhaps modified the buffer.
           */
          if (buffer_dirty(bh)) {
                  smp_mb();
                  if (buffer_dirty(bh))
                          return;
          }
  
          if (!test_set_buffer_dirty(bh)) {
                  struct page *page = bh->b_page;
                  if (!TestSetPageDirty(page)) {
                          struct address_space *mapping = page_mapping(page);
                          if (mapping)
                                  __set_page_dirty(page, mapping, 0);
                  }
          }
  }
  EXPORT_SYMBOL(mark_buffer_dirty);
  
  /*
   * Decrement a buffer_head's reference count.  If all buffers against a page
   * have zero reference count, are clean and unlocked, and if the page is clean
   * and unlocked then try_to_free_buffers() may strip the buffers from the page
   * in preparation for freeing it (sometimes, rarely, buffers are removed from
   * a page but it ends up not being freed, and buffers may later be reattached).
   */
  void __brelse(struct buffer_head * buf)
  {
          if (atomic_read(&buf->b_count)) {
                  put_bh(buf);
                  return;
          }
          WARN(1, KERN_ERR "VFS: brelse: Trying to free free buffer\n");
  }
  EXPORT_SYMBOL(__brelse);
  
  /*
   * bforget() is like brelse(), except it discards any
   * potentially dirty data.
   */
  void __bforget(struct buffer_head *bh)
  {
          clear_buffer_dirty(bh);
          if (bh->b_assoc_map) {
                  struct address_space *buffer_mapping = bh->b_page->mapping;
  
                  spin_lock(&buffer_mapping->private_lock);
                  list_del_init(&bh->b_assoc_buffers);
                  bh->b_assoc_map = NULL;
                  spin_unlock(&buffer_mapping->private_lock);
          }
          __brelse(bh);
  }
  EXPORT_SYMBOL(__bforget);
  
  static struct buffer_head *__bread_slow(struct buffer_head *bh)
  {
          lock_buffer(bh);
          if (buffer_uptodate(bh)) {
                  unlock_buffer(bh);
                  return bh;
          } else {
                  get_bh(bh);
                  bh->b_end_io = end_buffer_read_sync;
                  submit_bh(READ, bh);
                  wait_on_buffer(bh);
                  if (buffer_uptodate(bh))
                          return bh;
          }
          brelse(bh);
          return NULL;
  }
  
  /*
   * Per-cpu buffer LRU implementation.  To reduce the cost of __find_get_block().
   * The bhs[] array is sorted - newest buffer is at bhs[0].  Buffers have their
   * refcount elevated by one when they're in an LRU.  A buffer can only appear
   * once in a particular CPU's LRU.  A single buffer can be present in multiple
   * CPU's LRUs at the same time.
   *
   * This is a transparent caching front-end to sb_bread(), sb_getblk() and
   * sb_find_get_block().
   *
   * The LRUs themselves only need locking against invalidate_bh_lrus.  We use
   * a local interrupt disable for that.
   */
  
  #define BH_LRU_SIZE     8
  
  struct bh_lru {
          struct buffer_head *bhs[BH_LRU_SIZE];
  };
  
  static DEFINE_PER_CPU(struct bh_lru, bh_lrus) = {{ NULL }};
  
  #ifdef CONFIG_SMP
  #define bh_lru_lock()   local_irq_disable()
  #define bh_lru_unlock() local_irq_enable()
  #else
  #define bh_lru_lock()   preempt_disable()
  #define bh_lru_unlock() preempt_enable()
  #endif
  
  static inline void check_irqs_on(void)
  {
  #ifdef irqs_disabled
          BUG_ON(irqs_disabled());
  #endif
  }
  
  /*
   * The LRU management algorithm is dopey-but-simple.  Sorry.
   */
  static void bh_lru_install(struct buffer_head *bh)
  {
          struct buffer_head *evictee = NULL;
  
          check_irqs_on();
          bh_lru_lock();
          if (__this_cpu_read(bh_lrus.bhs[0]) != bh) {
                  struct buffer_head *bhs[BH_LRU_SIZE];
                  int in;
                  int out = 0;
  
                  get_bh(bh);
                  bhs[out++] = bh;
                  for (in = 0; in < BH_LRU_SIZE; in++) {
                          struct buffer_head *bh2 =
                                  __this_cpu_read(bh_lrus.bhs[in]);
  
                          if (bh2 == bh) {
                                  __brelse(bh2);
                          } else {
                                  if (out >= BH_LRU_SIZE) {
                                          BUG_ON(evictee != NULL);
                                          evictee = bh2;
                                  } else {
                                          bhs[out++] = bh2;
                                  }
                          }
                  }
                  while (out < BH_LRU_SIZE)
                          bhs[out++] = NULL;
                  memcpy(__this_cpu_ptr(&bh_lrus.bhs), bhs, sizeof(bhs));
          }
          bh_lru_unlock();
  
          if (evictee)
                  __brelse(evictee);
  }
  
  /*
   * Look up the bh in this cpu's LRU.  If it's there, move it to the head.
   */
  static struct buffer_head *
  lookup_bh_lru(struct block_device *bdev, sector_t block, unsigned size)
  {
          struct buffer_head *ret = NULL;
          unsigned int i;
  
          check_irqs_on();
          bh_lru_lock();
          for (i = 0; i < BH_LRU_SIZE; i++) {
                  struct buffer_head *bh = __this_cpu_read(bh_lrus.bhs[i]);
  
                  if (bh && bh->b_bdev == bdev &&
                                  bh->b_blocknr == block && bh->b_size == size) {
                          if (i) {
                                  while (i) {
                                          __this_cpu_write(bh_lrus.bhs[i],
                                                  __this_cpu_read(bh_lrus.bhs[i - 1]));
                                          i--;
                                  }
                                  __this_cpu_write(bh_lrus.bhs[0], bh);
                          }
                          get_bh(bh);
                          ret = bh;
                          break;
                  }
          }
          bh_lru_unlock();
          return ret;
  }
  
  /*
   * Perform a pagecache lookup for the matching buffer.  If it's there, refresh
   * it in the LRU and mark it as accessed.  If it is not present then return
   * NULL
   */
  struct buffer_head *
  __find_get_block(struct block_device *bdev, sector_t block, unsigned size)
  {
          struct buffer_head *bh = lookup_bh_lru(bdev, block, size);
  
          if (bh == NULL) {
                  bh = __find_get_block_slow(bdev, block);
                  if (bh)
                          bh_lru_install(bh);
          }
          if (bh)
                  touch_buffer(bh);
          return bh;
  }
  EXPORT_SYMBOL(__find_get_block);
  
  /*
   * __getblk will locate (and, if necessary, create) the buffer_head
   * which corresponds to the passed block_device, block and size. The
   * returned buffer has its reference count incremented.
   *
   * __getblk() will lock up the machine if grow_dev_page's try_to_free_buffers()
   * attempt is failing.  FIXME, perhaps?
   */
  struct buffer_head *
  __getblk(struct block_device *bdev, sector_t block, unsigned size)
  {
          struct buffer_head *bh = __find_get_block(bdev, block, size);
  
          might_sleep();
          if (bh == NULL)
                  bh = __getblk_slow(bdev, block, size);
          return bh;
  }
  EXPORT_SYMBOL(__getblk);
  
  /*
   * Do async read-ahead on a buffer..
   */
  void __breadahead(struct block_device *bdev, sector_t block, unsigned size)
  {
          struct buffer_head *bh = __getblk(bdev, block, size);
          if (likely(bh)) {
                  ll_rw_block(READA, 1, &bh);
                  brelse(bh);
          }
  }
  EXPORT_SYMBOL(__breadahead);
  
  /**
   *  __bread() - reads a specified block and returns the bh
   *  @bdev: the block_device to read from
   *  @block: number of block
   *  @size: size (in bytes) to read
   * 
   *  Reads a specified block, and returns buffer head that contains it.
   *  It returns NULL if the block was unreadable.
   */
  struct buffer_head *
  __bread(struct block_device *bdev, sector_t block, unsigned size)
  {
          struct buffer_head *bh = __getblk(bdev, block, size);
  
          if (likely(bh) && !buffer_uptodate(bh))
                  bh = __bread_slow(bh);
          return bh;
  }
  EXPORT_SYMBOL(__bread);
  
  /*
   * invalidate_bh_lrus() is called rarely - but not only at unmount.
   * This doesn't race because it runs in each cpu either in irq
   * or with preempt disabled.
   */
  static void invalidate_bh_lru(void *arg)
  {
          struct bh_lru *b = &get_cpu_var(bh_lrus);
          int i;
  
          for (i = 0; i < BH_LRU_SIZE; i++) {
                  brelse(b->bhs[i]);
                  b->bhs[i] = NULL;
          }
          put_cpu_var(bh_lrus);
  }
  
  static bool has_bh_in_lru(int cpu, void *dummy)
  {
          struct bh_lru *b = per_cpu_ptr(&bh_lrus, cpu);
          int i;
          
          for (i = 0; i < BH_LRU_SIZE; i++) {
                  if (b->bhs[i])
                          return 1;
          }
  
          return 0;
  }
  
  void invalidate_bh_lrus(void)
  {
          on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1, GFP_KERNEL);
  }
  EXPORT_SYMBOL_GPL(invalidate_bh_lrus);
  
  void set_bh_page(struct buffer_head *bh,
                  struct page *page, unsigned long offset)
  {
          bh->b_page = page;
          BUG_ON(offset >= PAGE_SIZE);
          if (PageHighMem(page))
                  /*
                   * This catches illegal uses and preserves the offset:
                   */
                  bh->b_data = (char *)(0 + offset);
          else
                  bh->b_data = page_address(page) + offset;
  }
  EXPORT_SYMBOL(set_bh_page);
  
  /*
   * Called when truncating a buffer on a page completely.
   */
  static void discard_buffer(struct buffer_head * bh)
  {
          lock_buffer(bh);
          clear_buffer_dirty(bh);
          bh->b_bdev = NULL;
          clear_buffer_mapped(bh);
          clear_buffer_req(bh);
          clear_buffer_new(bh);
          clear_buffer_delay(bh);
          clear_buffer_unwritten(bh);
          unlock_buffer(bh);
  }
  
  /**
   * block_invalidatepage - invalidate part or all of a buffer-backed page
   *
   * @page: the page which is affected
   * @offset: the index of the truncation point
   *
   * block_invalidatepage() is called when all or part of the page has become
   * invalidated by a truncate operation.
   *
   * block_invalidatepage() does not have to release all buffers, but it must
   * ensure that no dirty buffer is left outside @offset and that no I/O
   * is underway against any of the blocks which are outside the truncation
   * point.  Because the caller is about to free (and possibly reuse) those
   * blocks on-disk.
   */
  void block_invalidatepage(struct page *page, unsigned long offset)
  {
          struct buffer_head *head, *bh, *next;
          unsigned int curr_off = 0;
  
          BUG_ON(!PageLocked(page));
          if (!page_has_buffers(page))
                  goto out;
  
          head = page_buffers(page);
          bh = head;
          do {
                  unsigned int next_off = curr_off + bh->b_size;
                  next = bh->b_this_page;
  
                  /*
                   * is this block fully invalidated?
                   */
                  if (offset <= curr_off)
                          discard_buffer(bh);
                  curr_off = next_off;
                  bh = next;
          } while (bh != head);
  
          /*
           * We release buffers only if the entire page is being invalidated.
           * The get_block cached value has been unconditionally invalidated,
           * so real IO is not possible anymore.
           */
          if (offset == 0)
                  try_to_release_page(page, 0);
  out:
          return;
  }
  EXPORT_SYMBOL(block_invalidatepage);
  
  /*
   * We attach and possibly dirty the buffers atomically wrt
   * __set_page_dirty_buffers() via private_lock.  try_to_free_buffers
   * is already excluded via the page lock.
   */
  void create_empty_buffers(struct page *page,
                          unsigned long blocksize, unsigned long b_state)
  {
          struct buffer_head *bh, *head, *tail;
  
          head = alloc_page_buffers(page, blocksize, 1);
          bh = head;
          do {
                  bh->b_state |= b_state;
                  tail = bh;
                  bh = bh->b_this_page;
          } while (bh);
          tail->b_this_page = head;
  
          spin_lock(&page->mapping->private_lock);
          if (PageUptodate(page) || PageDirty(page)) {
                  bh = head;
                  do {
                          if (PageDirty(page))
                                  set_buffer_dirty(bh);
                          if (PageUptodate(page))
                                  set_buffer_uptodate(bh);
                          bh = bh->b_this_page;
                  } while (bh != head);
          }
          attach_page_buffers(page, head);
          spin_unlock(&page->mapping->private_lock);
  }
  EXPORT_SYMBOL(create_empty_buffers);
  
  /*
   * We are taking a block for data and we don't want any output from any
   * buffer-cache aliases starting from return from that function and
   * until the moment when something will explicitly mark the buffer
   * dirty (hopefully that will not happen until we will free that block ;-)
   * We don't even need to mark it not-uptodate - nobody can expect
   * anything from a newly allocated buffer anyway. We used to used
   * unmap_buffer() for such invalidation, but that was wrong. We definitely
   * don't want to mark the alias unmapped, for example - it would confuse
   * anyone who might pick it with bread() afterwards...
   *
   * Also..  Note that bforget() doesn't lock the buffer.  So there can
   * be writeout I/O going on against recently-freed buffers.  We don't
   * wait on that I/O in bforget() - it's more efficient to wait on the I/O
   * only if we really need to.  That happens here.
   */
  void unmap_underlying_metadata(struct block_device *bdev, sector_t block)
  {
          struct buffer_head *old_bh;
  
          might_sleep();
  
          old_bh = __find_get_block_slow(bdev, block);
          if (old_bh) {
                  clear_buffer_dirty(old_bh);
                  wait_on_buffer(old_bh);
                  clear_buffer_req(old_bh);
                  __brelse(old_bh);
          }
  }
  EXPORT_SYMBOL(unmap_underlying_metadata);
  
  /*
   * Size is a power-of-two in the range 512..PAGE_SIZE,
   * and the case we care about most is PAGE_SIZE.
   *
   * So this *could* possibly be written with those
   * constraints in mind (relevant mostly if some
   * architecture has a slow bit-scan instruction)
   */
  static inline int block_size_bits(unsigned int blocksize)
  {
          return ilog2(blocksize);
  }
  
  static struct buffer_head *create_page_buffers(struct page *page, struct inode *inode, unsigned int b_state)
  {
          BUG_ON(!PageLocked(page));
  
          if (!page_has_buffers(page))
                  create_empty_buffers(page, 1 << ACCESS_ONCE(inode->i_blkbits), b_state);
          return page_buffers(page);
  }
  
  /*
   * NOTE! All mapped/uptodate combinations are valid:
   *
   *      Mapped  Uptodate        Meaning
   *
   *      No      No              "unknown" - must do get_block()
   *      No      Yes             "hole" - zero-filled
   *      Yes     No              "allocated" - allocated on disk, not read in
   *      Yes     Yes             "valid" - allocated and up-to-date in memory.
   *
   * "Dirty" is valid only with the last case (mapped+uptodate).
   */
  
  /*
   * While block_write_full_page is writing back the dirty buffers under
   * the page lock, whoever dirtied the buffers may decide to clean them
   * again at any time.  We handle that by only looking at the buffer
   * state inside lock_buffer().
   *
   * If block_write_full_page() is called for regular writeback
   * (wbc->sync_mode == WB_SYNC_NONE) then it will redirty a page which has a
   * locked buffer.   This only can happen if someone has written the buffer
   * directly, with submit_bh().  At the address_space level PageWriteback
   * prevents this contention from occurring.
   *
   * If block_write_full_page() is called with wbc->sync_mode ==
   * WB_SYNC_ALL, the writes are posted using WRITE_SYNC; this
   * causes the writes to be flagged as synchronous writes.
   */
  static int __block_write_full_page(struct inode *inode, struct page *page,
                          get_block_t *get_block, struct writeback_control *wbc,
                          bh_end_io_t *handler)
  {
          int err;
          sector_t block;
          sector_t last_block;
          struct buffer_head *bh, *head;
          unsigned int blocksize, bbits;
          int nr_underway = 0;
          int write_op = (wbc->sync_mode == WB_SYNC_ALL ?
                          WRITE_SYNC : WRITE);
  
          head = create_page_buffers(page, inode,
                                          (1 << BH_Dirty)|(1 << BH_Uptodate));
  
          /*
           * Be very careful.  We have no exclusion from __set_page_dirty_buffers
           * here, and the (potentially unmapped) buffers may become dirty at
           * any time.  If a buffer becomes dirty here after we've inspected it
           * then we just miss that fact, and the page stays dirty.
           *
           * Buffers outside i_size may be dirtied by __set_page_dirty_buffers;
           * handle that here by just cleaning them.
           */
  
          bh = head;
          blocksize = bh->b_size;
          bbits = block_size_bits(blocksize);
  
          block = (sector_t)page->index << (PAGE_CACHE_SHIFT - bbits);
          last_block = (i_size_read(inode) - 1) >> bbits;
  
          /*
           * Get all the dirty buffers mapped to disk addresses and
           * handle any aliases from the underlying blockdev's mapping.
           */
          do {
                  if (block > last_block) {
                          /*
                           * mapped buffers outside i_size will occur, because
                           * this page can be outside i_size when there is a
                           * truncate in progress.
                           */
                          /*
                           * The buffer was zeroed by block_write_full_page()
                           */
                          clear_buffer_dirty(bh);
                          set_buffer_uptodate(bh);
                  } else if ((!buffer_mapped(bh) || buffer_delay(bh)) &&
                             buffer_dirty(bh)) {
                          WARN_ON(bh->b_size != blocksize);
                          err = get_block(inode, block, bh, 1);
                          if (err)
                                  goto recover;
                          clear_buffer_delay(bh);
                          if (buffer_new(bh)) {
                                  /* blockdev mappings never come here */
                                  clear_buffer_new(bh);
                                  unmap_underlying_metadata(bh->b_bdev,
                                                          bh->b_blocknr);
                          }
                  }
                  bh = bh->b_this_page;
                  block++;
          } while (bh != head);
  
          do {
                  if (!buffer_mapped(bh))
                          continue;
                  /*
                   * If it's a fully non-blocking write attempt and we cannot
                   * lock the buffer then redirty the page.  Note that this can
                   * potentially cause a busy-wait loop from writeback threads
                   * and kswapd activity, but those code paths have their own
                   * higher-level throttling.
                   */
                  if (wbc->sync_mode != WB_SYNC_NONE) {
                          lock_buffer(bh);
                  } else if (!trylock_buffer(bh)) {
                          redirty_page_for_writepage(wbc, page);
                          continue;
                  }
                  if (test_clear_buffer_dirty(bh)) {
                          mark_buffer_async_write_endio(bh, handler);
                  } else {
                          unlock_buffer(bh);
                  }
          } while ((bh = bh->b_this_page) != head);
  
          /*
           * The page and its buffers are protected by PageWriteback(), so we can
           * drop the bh refcounts early.
           */
          BUG_ON(PageWriteback(page));
          set_page_writeback(page);
  
          do {
                  struct buffer_head *next = bh->b_this_page;
                  if (buffer_async_write(bh)) {
                          submit_bh(write_op, bh);
                          nr_underway++;
                  }
                  bh = next;
          } while (bh != head);
          unlock_page(page);
  
          err = 0;
  done:
          if (nr_underway == 0) {
                  /*
                   * The page was marked dirty, but the buffers were
                   * clean.  Someone wrote them back by hand with
                   * ll_rw_block/submit_bh.  A rare case.
                   */
                  end_page_writeback(page);
  
                  /*
                   * The page and buffer_heads can be released at any time from
                   * here on.
                   */
          }
          return err;
  
  recover:
          /*
           * ENOSPC, or some other error.  We may already have added some
           * blocks to the file, so we need to write these out to avoid
           * exposing stale data.
           * The page is currently locked and not marked for writeback
           */
          bh = head;
          /* Recovery: lock and submit the mapped buffers */
          do {
                  if (buffer_mapped(bh) && buffer_dirty(bh) &&
                      !buffer_delay(bh)) {
                          lock_buffer(bh);
                          mark_buffer_async_write_endio(bh, handler);
                  } else {
                          /*
                           * The buffer may have been set dirty during
                           * attachment to a dirty page.
                           */
                          clear_buffer_dirty(bh);
                  }
          } while ((bh = bh->b_this_page) != head);
          SetPageError(page);
          BUG_ON(PageWriteback(page));
          mapping_set_error(page->mapping, err);
          set_page_writeback(page);
          do {
                  struct buffer_head *next = bh->b_this_page;
                  if (buffer_async_write(bh)) {
                          clear_buffer_dirty(bh);
                          submit_bh(write_op, bh);
                          nr_underway++;
                  }
                  bh = next;
          } while (bh != head);
          unlock_page(page);
          goto done;
  }
  
  /*
   * If a page has any new buffers, zero them out here, and mark them uptodate
   * and dirty so they'll be written out (in order to prevent uninitialised
   * block data from leaking). And clear the new bit.
   */
  void page_zero_new_buffers(struct page *page, unsigned from, unsigned to)
  {
          unsigned int block_start, block_end;
          struct buffer_head *head, *bh;
  
          BUG_ON(!PageLocked(page));
          if (!page_has_buffers(page))
                  return;
  
          bh = head = page_buffers(page);
          block_start = 0;
          do {
                  block_end = block_start + bh->b_size;
  
                  if (buffer_new(bh)) {
                          if (block_end > from && block_start < to) {
                                  if (!PageUptodate(page)) {
                                          unsigned start, size;
  
                                          start = max(from, block_start);
                                          size = min(to, block_end) - start;
  
                                          zero_user(page, start, size);
                                          set_buffer_uptodate(bh);
                                  }
  
                                  clear_buffer_new(bh);
                                  mark_buffer_dirty(bh);
                          }
                  }
  
                  block_start = block_end;
                  bh = bh->b_this_page;
          } while (bh != head);
  }
  EXPORT_SYMBOL(page_zero_new_buffers);
  
  int __block_write_begin(struct page *page, loff_t pos, unsigned len,
                  get_block_t *get_block)
  {
          unsigned from = pos & (PAGE_CACHE_SIZE - 1);
          unsigned to = from + len;
          struct inode *inode = page->mapping->host;
          unsigned block_start, block_end;
          sector_t block;
          int err = 0;
          unsigned blocksize, bbits;
          struct buffer_head *bh, *head, *wait[2], **wait_bh=wait;
  
          BUG_ON(!PageLocked(page));
          BUG_ON(from > PAGE_CACHE_SIZE);
          BUG_ON(to > PAGE_CACHE_SIZE);
          BUG_ON(from > to);
  
          head = create_page_buffers(page, inode, 0);
          blocksize = head->b_size;
          bbits = block_size_bits(blocksize);
  
          block = (sector_t)page->index << (PAGE_CACHE_SHIFT - bbits);
  
          for(bh = head, block_start = 0; bh != head || !block_start;
              block++, block_start=block_end, bh = bh->b_this_page) {
                  block_end = block_start + blocksize;
                  if (block_end <= from || block_start >= to) {
                          if (PageUptodate(page)) {
                                  if (!buffer_uptodate(bh))
                                          set_buffer_uptodate(bh);
                          }
                          continue;
                  }
                  if (buffer_new(bh))
                          clear_buffer_new(bh);
                  if (!buffer_mapped(bh)) {
                          WARN_ON(bh->b_size != blocksize);
                          err = get_block(inode, block, bh, 1);
                          if (err)
                                  break;
                          if (buffer_new(bh)) {
                                  unmap_underlying_metadata(bh->b_bdev,
                                                          bh->b_blocknr);
                                  if (PageUptodate(page)) {
                                          clear_buffer_new(bh);
                                          set_buffer_uptodate(bh);
                                          mark_buffer_dirty(bh);
                                          continue;
                                  }
                                  if (block_end > to || block_start < from)
                                          zero_user_segments(page,
                                                  to, block_end,
                                                  block_start, from);
                                  continue;
                          }
                  }
                  if (PageUptodate(page)) {
                          if (!buffer_uptodate(bh))
                                  set_buffer_uptodate(bh);
                          continue; 
                  }
                  if (!buffer_uptodate(bh) && !buffer_delay(bh) &&
                      !buffer_unwritten(bh) &&
                       (block_start < from || block_end > to)) {
                          ll_rw_block(READ, 1, &bh);
                          *wait_bh++=bh;
                  }
          }
          /*
           * If we issued read requests - let them complete.
           */
          while(wait_bh > wait) {
                  wait_on_buffer(*--wait_bh);
                  if (!buffer_uptodate(*wait_bh))
                          err = -EIO;
          }
          if (unlikely(err))
                  page_zero_new_buffers(page, from, to);
          return err;
  }
  EXPORT_SYMBOL(__block_write_begin);
  
  static int __block_commit_write(struct inode *inode, struct page *page,
                  unsigned from, unsigned to)
  {
          unsigned block_start, block_end;
          int partial = 0;
          unsigned blocksize;
          struct buffer_head *bh, *head;
  
          bh = head = page_buffers(page);
          blocksize = bh->b_size;
  
          block_start = 0;
          do {
                  block_end = block_start + blocksize;
                  if (block_end <= from || block_start >= to) {
                          if (!buffer_uptodate(bh))
                                  partial = 1;
                  } else {
                          set_buffer_uptodate(bh);
                          mark_buffer_dirty(bh);
                  }
                  clear_buffer_new(bh);
  
                  block_start = block_end;
                  bh = bh->b_this_page;
          } while (bh != head);
  
          /*
           * If this is a partial write which happened to make all buffers
           * uptodate then we can optimize away a bogus readpage() for
           * the next read(). Here we 'discover' whether the page went
           * uptodate as a result of this (potentially partial) write.
           */
          if (!partial)
                  SetPageUptodate(page);
          return 0;
  }
  
  /*
   * block_write_begin takes care of the basic task of block allocation and
   * bringing partial write blocks uptodate first.
   *
   * The filesystem needs to handle block truncation upon failure.
   */
  int block_write_begin(struct address_space *mapping, loff_t pos, unsigned len,
                  unsigned flags, struct page **pagep, get_block_t *get_block)
  {
          pgoff_t index = pos >> PAGE_CACHE_SHIFT;
          struct page *page;
          int status;
  
          page = grab_cache_page_write_begin(mapping, index, flags);
          if (!page)
                  return -ENOMEM;
  
          status = __block_write_begin(page, pos, len, get_block);
          if (unlikely(status)) {
                  unlock_page(page);
                  page_cache_release(page);
                  page = NULL;
          }
  
          *pagep = page;
          return status;
  }
  EXPORT_SYMBOL(block_write_begin);
  
  int block_write_end(struct file *file, struct address_space *mapping,
                          loff_t pos, unsigned len, unsigned copied,
                          struct page *page, void *fsdata)
  {
          struct inode *inode = mapping->host;
          unsigned start;
  
          start = pos & (PAGE_CACHE_SIZE - 1);
  
          if (unlikely(copied < len)) {
                  /*
                   * The buffers that were written will now be uptodate, so we
                   * don't have to worry about a readpage reading them and
                   * overwriting a partial write. However if we have encountered
                   * a short write and only partially written into a buffer, it
                   * will not be marked uptodate, so a readpage might come in and
                   * destroy our partial write.
                   *
                   * Do the simplest thing, and just treat any short write to a
                   * non uptodate page as a zero-length write, and force the
                   * caller to redo the whole thing.
                   */
                  if (!PageUptodate(page))
                          copied = 0;
  
                  page_zero_new_buffers(page, start+copied, start+len);
          }
          flush_dcache_page(page);
  
          /* This could be a short (even 0-length) commit */
          __block_commit_write(inode, page, start, start+copied);
  
          return copied;
  }
  EXPORT_SYMBOL(block_write_end);
  
  int generic_write_end(struct file *file, struct address_space *mapping,
                          loff_t pos, unsigned len, unsigned copied,
                          struct page *page, void *fsdata)
  {
          struct inode *inode = mapping->host;
          int i_size_changed = 0;
  
          copied = block_write_end(file, mapping, pos, len, copied, page, fsdata);
  
          /*
           * No need to use i_size_read() here, the i_size
           * cannot change under us because we hold i_mutex.
           *
           * But it's important to update i_size while still holding page lock:
           * page writeout could otherwise come in and zero beyond i_size.
           */
          if (pos+copied > inode->i_size) {
                  i_size_write(inode, pos+copied);
                  i_size_changed = 1;
          }
  
          unlock_page(page);
          page_cache_release(page);
  
          /*
           * Don't mark the inode dirty under page lock. First, it unnecessarily
           * makes the holding time of page lock longer. Second, it forces lock
           * ordering of page lock and transaction start for journaling
           * filesystems.
           */
          if (i_size_changed)
                  mark_inode_dirty(inode);
  
          return copied;
  }
  EXPORT_SYMBOL(generic_write_end);
  
  /*
   * block_is_partially_uptodate checks whether buffers within a page are
   * uptodate or not.
   *
   * Returns true if all buffers which correspond to a file portion
   * we want to read are uptodate.
   */
  int block_is_partially_uptodate(struct page *page, read_descriptor_t *desc,
                                          unsigned long from)
  {
          unsigned block_start, block_end, blocksize;
          unsigned to;
          struct buffer_head *bh, *head;
          int ret = 1;
  
          if (!page_has_buffers(page))
                  return 0;
  
          head = page_buffers(page);
          blocksize = head->b_size;
          to = min_t(unsigned, PAGE_CACHE_SIZE - from, desc->count);
          to = from + to;
          if (from < blocksize && to > PAGE_CACHE_SIZE - blocksize)
                  return 0;
  
          bh = head;
          block_start = 0;
          do {
                  block_end = block_start + blocksize;
                  if (block_end > from && block_start < to) {
                          if (!buffer_uptodate(bh)) {
                                  ret = 0;
                                  break;
                          }
                          if (block_end >= to)
                                  break;
                  }
                  block_start = block_end;
                  bh = bh->b_this_page;
          } while (bh != head);
  
          return ret;
  }
  EXPORT_SYMBOL(block_is_partially_uptodate);
  
  /*
   * Generic "read page" function for block devices that have the normal
   * get_block functionality. This is most of the block device filesystems.
   * Reads the page asynchronously --- the unlock_buffer() and
   * set/clear_buffer_uptodate() functions propagate buffer state into the
   * page struct once IO has completed.
   */
  int block_read_full_page(struct page *page, get_block_t *get_block)
  {
          struct inode *inode = page->mapping->host;
          sector_t iblock, lblock;
          struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
          unsigned int blocksize, bbits;
          int nr, i;
          int fully_mapped = 1;
  
          head = create_page_buffers(page, inode, 0);
          blocksize = head->b_size;
          bbits = block_size_bits(blocksize);
  
          iblock = (sector_t)page->index << (PAGE_CACHE_SHIFT - bbits);
          lblock = (i_size_read(inode)+blocksize-1) >> bbits;
          bh = head;
          nr = 0;
          i = 0;
  
          do {
                  if (buffer_uptodate(bh))
                          continue;
  
                  if (!buffer_mapped(bh)) {
                          int err = 0;
  
                          fully_mapped = 0;
                          if (iblock < lblock) {
                                  WARN_ON(bh->b_size != blocksize);
                                  err = get_block(inode, iblock, bh, 0);
                                  if (err)
                                          SetPageError(page);
                          }
                          if (!buffer_mapped(bh)) {
                                  zero_user(page, i * blocksize, blocksize);
                                  if (!err)
                                          set_buffer_uptodate(bh);
                                  continue;
                          }
                          /*
                           * get_block() might have updated the buffer
                           * synchronously
                           */
                          if (buffer_uptodate(bh))
                                  continue;
                  }
                  arr[nr++] = bh;
          } while (i++, iblock++, (bh = bh->b_this_page) != head);
  
          if (fully_mapped)
                  SetPageMappedToDisk(page);
  
          if (!nr) {
                  /*
                   * All buffers are uptodate - we can set the page uptodate
                   * as well. But not if get_block() returned an error.
                   */
                  if (!PageError(page))
                          SetPageUptodate(page);
                  unlock_page(page);
                  return 0;
          }
  
          /* Stage two: lock the buffers */
          for (i = 0; i < nr; i++) {
                  bh = arr[i];
                  lock_buffer(bh);
                  mark_buffer_async_read(bh);
          }
  
          /*
           * Stage 3: start the IO.  Check for uptodateness
           * inside the buffer lock in case another process reading
           * the underlying blockdev brought it uptodate (the sct fix).
           */
          for (i = 0; i < nr; i++) {
                  bh = arr[i];
                  if (buffer_uptodate(bh))
                          end_buffer_async_read(bh, 1);
                  else
                          submit_bh(READ, bh);
          }
          return 0;
  }
  EXPORT_SYMBOL(block_read_full_page);
  
  /* utility function for filesystems that need to do work on expanding
   * truncates.  Uses filesystem pagecache writes to allow the filesystem to
   * deal with the hole.  
   */
  int generic_cont_expand_simple(struct inode *inode, loff_t size)
  {
          struct address_space *mapping = inode->i_mapping;
          struct page *page;
          void *fsdata;
          int err;
  
          err = inode_newsize_ok(inode, size);
          if (err)
                  goto out;
  
          err = pagecache_write_begin(NULL, mapping, size, 0,
                                  AOP_FLAG_UNINTERRUPTIBLE|AOP_FLAG_CONT_EXPAND,
                                  &page, &fsdata);
          if (err)
                  goto out;
  
          err = pagecache_write_end(NULL, mapping, size, 0, 0, page, fsdata);
          BUG_ON(err > 0);
  
  out:
          return err;
  }
  EXPORT_SYMBOL(generic_cont_expand_simple);
  
  static int cont_expand_zero(struct file *file, struct address_space *mapping,
                              loff_t pos, loff_t *bytes)
  {
          struct inode *inode = mapping->host;
          unsigned blocksize = 1 << inode->i_blkbits;
          struct page *page;
          void *fsdata;
          pgoff_t index, curidx;
          loff_t curpos;
          unsigned zerofrom, offset, len;
          int err = 0;
  
          index = pos >> PAGE_CACHE_SHIFT;
          offset = pos & ~PAGE_CACHE_MASK;
  
          while (index > (curidx = (curpos = *bytes)>>PAGE_CACHE_SHIFT)) {
                  zerofrom = curpos & ~PAGE_CACHE_MASK;
                  if (zerofrom & (blocksize-1)) {
                          *bytes |= (blocksize-1);
                          (*bytes)++;
                  }
                  len = PAGE_CACHE_SIZE - zerofrom;
  
                  err = pagecache_write_begin(file, mapping, curpos, len,
                                                  AOP_FLAG_UNINTERRUPTIBLE,
                                                  &page, &fsdata);
                  if (err)
                          goto out;
                  zero_user(page, zerofrom, len);
                  err = pagecache_write_end(file, mapping, curpos, len, len,
                                                  page, fsdata);
                  if (err < 0)
                          goto out;
                  BUG_ON(err != len);
                  err = 0;
  
                  balance_dirty_pages_ratelimited(mapping);
          }
  
          /* page covers the boundary, find the boundary offset */
          if (index == curidx) {
                  zerofrom = curpos & ~PAGE_CACHE_MASK;
                  /* if we will expand the thing last block will be filled */
                  if (offset <= zerofrom) {
                          goto out;
                  }
                  if (zerofrom & (blocksize-1)) {
                          *bytes |= (blocksize-1);
                          (*bytes)++;
                  }
                  len = offset - zerofrom;
  
                  err = pagecache_write_begin(file, mapping, curpos, len,
                                                  AOP_FLAG_UNINTERRUPTIBLE,
                                                  &page, &fsdata);
                  if (err)
                          goto out;
                  zero_user(page, zerofrom, len);
                  err = pagecache_write_end(file, mapping, curpos, len, len,
                                                  page, fsdata);
                  if (err < 0)
                          goto out;
                  BUG_ON(err != len);
                  err = 0;
          }
  out:
          return err;
  }
  
  /*
   * For moronic filesystems that do not allow holes in file.
   * We may have to extend the file.
   */
  int cont_write_begin(struct file *file, struct address_space *mapping,
                          loff_t pos, unsigned len, unsigned flags,
                          struct page **pagep, void **fsdata,
                          get_block_t *get_block, loff_t *bytes)
  {
          struct inode *inode = mapping->host;
          unsigned blocksize = 1 << inode->i_blkbits;
          unsigned zerofrom;
          int err;
  
          err = cont_expand_zero(file, mapping, pos, bytes);
          if (err)
                  return err;
  
          zerofrom = *bytes & ~PAGE_CACHE_MASK;
          if (pos+len > *bytes && zerofrom & (blocksize-1)) {
                  *bytes |= (blocksize-1);
                  (*bytes)++;
          }
  
          return block_write_begin(mapping, pos, len, flags, pagep, get_block);
  }
  EXPORT_SYMBOL(cont_write_begin);
  
  int block_commit_write(struct page *page, unsigned from, unsigned to)
  {
          struct inode *inode = page->mapping->host;
          __block_commit_write(inode,page,from,to);
          return 0;
  }
  EXPORT_SYMBOL(block_commit_write);
  
  /*
   * block_page_mkwrite() is not allowed to change the file size as it gets
   * called from a page fault handler when a page is first dirtied. Hence we must
   * be careful to check for EOF conditions here. We set the page up correctly
   * for a written page which means we get ENOSPC checking when writing into
   * holes and correct delalloc and unwritten extent mapping on filesystems that
   * support these features.
   *
   * We are not allowed to take the i_mutex here so we have to play games to
   * protect against truncate races as the page could now be beyond EOF.  Because
   * truncate writes the inode size before removing pages, once we have the
   * page lock we can determine safely if the page is beyond EOF. If it is not
   * beyond EOF, then the page is guaranteed safe against truncation until we
   * unlock the page.
   *
   * Direct callers of this function should protect against filesystem freezing
   * using sb_start_write() - sb_end_write() functions.
   */
  int __block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
                           get_block_t get_block)
  {
          struct page *page = vmf->page;
          struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
          unsigned long end;
          loff_t size;
          int ret;
  
          lock_page(page);
          size = i_size_read(inode);
          if ((page->mapping != inode->i_mapping) ||
              (page_offset(page) > size)) {
                  /* We overload EFAULT to mean page got truncated */
                  ret = -EFAULT;
                  goto out_unlock;
          }
  
          /* page is wholly or partially inside EOF */
          if (((page->index + 1) << PAGE_CACHE_SHIFT) > size)
                  end = size & ~PAGE_CACHE_MASK;
          else
                  end = PAGE_CACHE_SIZE;
  
          ret = __block_write_begin(page, 0, end, get_block);
          if (!ret)
                  ret = block_commit_write(page, 0, end);
  
          if (unlikely(ret < 0))
                  goto out_unlock;
          set_page_dirty(page);
          wait_on_page_writeback(page);
          return 0;
  out_unlock:
          unlock_page(page);
          return ret;
  }
  EXPORT_SYMBOL(__block_page_mkwrite);
  
  int block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
                     get_block_t get_block)
  {
          int ret;
          struct super_block *sb = vma->vm_file->f_path.dentry->d_inode->i_sb;
  
          sb_start_pagefault(sb);
  
          /*
           * Update file times before taking page lock. We may end up failing the
           * fault so this update may be superfluous but who really cares...
           */
          file_update_time(vma->vm_file);
  
          ret = __block_page_mkwrite(vma, vmf, get_block);
          sb_end_pagefault(sb);
          return block_page_mkwrite_return(ret);
  }
  EXPORT_SYMBOL(block_page_mkwrite);
  
  /*
   * nobh_write_begin()'s prereads are special: the buffer_heads are freed
   * immediately, while under the page lock.  So it needs a special end_io
   * handler which does not touch the bh after unlocking it.
   */
  static void end_buffer_read_nobh(struct buffer_head *bh, int uptodate)
  {
          __end_buffer_read_notouch(bh, uptodate);
  }
  
  /*
   * Attach the singly-linked list of buffers created by nobh_write_begin, to
   * the page (converting it to circular linked list and taking care of page
   * dirty races).
   */
  static void attach_nobh_buffers(struct page *page, struct buffer_head *head)
  {
          struct buffer_head *bh;
  
          BUG_ON(!PageLocked(page));
  
          spin_lock(&page->mapping->private_lock);
          bh = head;
          do {
                  if (PageDirty(page))
                          set_buffer_dirty(bh);
                  if (!bh->b_this_page)
                          bh->b_this_page = head;
                  bh = bh->b_this_page;
          } while (bh != head);
          attach_page_buffers(page, head);
          spin_unlock(&page->mapping->private_lock);
  }
  
  /*
   * On entry, the page is fully not uptodate.
   * On exit the page is fully uptodate in the areas outside (from,to)
   * The filesystem needs to handle block truncation upon failure.
   */
  int nobh_write_begin(struct address_space *mapping,
                          loff_t pos, unsigned len, unsigned flags,
                          struct page **pagep, void **fsdata,
                          get_block_t *get_block)
  {
          struct inode *inode = mapping->host;
          const unsigned blkbits = inode->i_blkbits;
          const unsigned blocksize = 1 << blkbits;
          struct buffer_head *head, *bh;
          struct page *page;
          pgoff_t index;
          unsigned from, to;
          unsigned block_in_page;
          unsigned block_start, block_end;
          sector_t block_in_file;
          int nr_reads = 0;
          int ret = 0;
          int is_mapped_to_disk = 1;
  
          index = pos >> PAGE_CACHE_SHIFT;
          from = pos & (PAGE_CACHE_SIZE - 1);
          to = from + len;
  
          page = grab_cache_page_write_begin(mapping, index, flags);
          if (!page)
                  return -ENOMEM;
          *pagep = page;
          *fsdata = NULL;
  
          if (page_has_buffers(page)) {
                  ret = __block_write_begin(page, pos, len, get_block);
                  if (unlikely(ret))
                          goto out_release;
                  return ret;
          }
  
          if (PageMappedToDisk(page))
                  return 0;
  
          /*
           * Allocate buffers so that we can keep track of state, and potentially
           * attach them to the page if an error occurs. In the common case of
           * no error, they will just be freed again without ever being attached
           * to the page (which is all OK, because we're under the page lock).
           *
           * Be careful: the buffer linked list is a NULL terminated one, rather
           * than the circular one we're used to.
           */
          head = alloc_page_buffers(page, blocksize, 0);
          if (!head) {
                  ret = -ENOMEM;
                  goto out_release;
          }
  
          block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
  
          /*
           * We loop across all blocks in the page, whether or not they are
           * part of the affected region.  This is so we can discover if the
           * page is fully mapped-to-disk.
           */
          for (block_start = 0, block_in_page = 0, bh = head;
                    block_start < PAGE_CACHE_SIZE;
                    block_in_page++, block_start += blocksize, bh = bh->b_this_page) {
                  int create;
  
                  block_end = block_start + blocksize;
                  bh->b_state = 0;
                  create = 1;
                  if (block_start >= to)
                          create = 0;
                  ret = get_block(inode, block_in_file + block_in_page,
                                          bh, create);
                  if (ret)
                          goto failed;
                  if (!buffer_mapped(bh))
                          is_mapped_to_disk = 0;
                  if (buffer_new(bh))
                          unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
                  if (PageUptodate(page)) {
                          set_buffer_uptodate(bh);
                          continue;
                  }
                  if (buffer_new(bh) || !buffer_mapped(bh)) {
                          zero_user_segments(page, block_start, from,
                                                          to, block_end);
                          continue;
                  }
                  if (buffer_uptodate(bh))
                          continue;       /* reiserfs does this */
                  if (block_start < from || block_end > to) {
                          lock_buffer(bh);
                          bh->b_end_io = end_buffer_read_nobh;
                          submit_bh(READ, bh);
                          nr_reads++;
                  }
          }
  
          if (nr_reads) {
                  /*
                   * The page is locked, so these buffers are protected from
                   * any VM or truncate activity.  Hence we don't need to care
                   * for the buffer_head refcounts.
                   */
                  for (bh = head; bh; bh = bh->b_this_page) {
                          wait_on_buffer(bh);
                          if (!buffer_uptodate(bh))
                                  ret = -EIO;
                  }
                  if (ret)
                          goto failed;
          }
  
          if (is_mapped_to_disk)
                  SetPageMappedToDisk(page);
  
          *fsdata = head; /* to be released by nobh_write_end */
  
          return 0;
  
  failed:
          BUG_ON(!ret);
          /*
           * Error recovery is a bit difficult. We need to zero out blocks that
           * were newly allocated, and dirty them to ensure they get written out.
           * Buffers need to be attached to the page at this point, otherwise
           * the handling of potential IO errors during writeout would be hard
           * (could try doing synchronous writeout, but what if that fails too?)
           */
          attach_nobh_buffers(page, head);
          page_zero_new_buffers(page, from, to);
  
  out_release:
          unlock_page(page);
          page_cache_release(page);
          *pagep = NULL;
  
          return ret;
  }
  EXPORT_SYMBOL(nobh_write_begin);
  
  int nobh_write_end(struct file *file, struct address_space *mapping,
                          loff_t pos, unsigned len, unsigned copied,
                          struct page *page, void *fsdata)
  {
          struct inode *inode = page->mapping->host;
          struct buffer_head *head = fsdata;
          struct buffer_head *bh;
          BUG_ON(fsdata != NULL && page_has_buffers(page));
  
          if (unlikely(copied < len) && head)
                  attach_nobh_buffers(page, head);
          if (page_has_buffers(page))
                  return generic_write_end(file, mapping, pos, len,
                                          copied, page, fsdata);
  
          SetPageUptodate(page);
          set_page_dirty(page);
          if (pos+copied > inode->i_size) {
                  i_size_write(inode, pos+copied);
                  mark_inode_dirty(inode);
          }
  
          unlock_page(page);
          page_cache_release(page);
  
          while (head) {
                  bh = head;
                  head = head->b_this_page;
                  free_buffer_head(bh);
          }
  
          return copied;
  }
  EXPORT_SYMBOL(nobh_write_end);
  
  /*
   * nobh_writepage() - based on block_full_write_page() except
   * that it tries to operate without attaching bufferheads to
   * the page.
   */
  int nobh_writepage(struct page *page, get_block_t *get_block,
                          struct writeback_control *wbc)
  {
          struct inode * const inode = page->mapping->host;
          loff_t i_size = i_size_read(inode);
          const pgoff_t end_index = i_size >> PAGE_CACHE_SHIFT;
          unsigned offset;
          int ret;
  
          /* Is the page fully inside i_size? */
          if (page->index < end_index)
                  goto out;
  
          /* Is the page fully outside i_size? (truncate in progress) */
          offset = i_size & (PAGE_CACHE_SIZE-1);
          if (page->index >= end_index+1 || !offset) {
                  /*
                   * The page may have dirty, unmapped buffers.  For example,
                   * they may have been added in ext3_writepage().  Make them
                   * freeable here, so the page does not leak.
                   */
  #if 0
                  /* Not really sure about this  - do we need this ? */
                  if (page->mapping->a_ops->invalidatepage)
                          page->mapping->a_ops->invalidatepage(page, offset);
  #endif
                  unlock_page(page);
                  return 0; /* don't care */
          }
  
          /*
           * The page straddles i_size.  It must be zeroed out on each and every
           * writepage invocation because it may be mmapped.  "A file is mapped
           * in multiples of the page size.  For a file that is not a multiple of
           * the  page size, the remaining memory is zeroed when mapped, and
           * writes to that region are not written out to the file."
           */
          zero_user_segment(page, offset, PAGE_CACHE_SIZE);
  out:
          ret = mpage_writepage(page, get_block, wbc);
          if (ret == -EAGAIN)
                  ret = __block_write_full_page(inode, page, get_block, wbc,
                                                end_buffer_async_write);
          return ret;
  }
  EXPORT_SYMBOL(nobh_writepage);
  
  int nobh_truncate_page(struct address_space *mapping,
                          loff_t from, get_block_t *get_block)
  {
          pgoff_t index = from >> PAGE_CACHE_SHIFT;
          unsigned offset = from & (PAGE_CACHE_SIZE-1);
          unsigned blocksize;
          sector_t iblock;
          unsigned length, pos;
          struct inode *inode = mapping->host;
          struct page *page;
          struct buffer_head map_bh;
          int err;
  
          blocksize = 1 << inode->i_blkbits;
          length = offset & (blocksize - 1);
  
          /* Block boundary? Nothing to do */
          if (!length)
                  return 0;
  
          length = blocksize - length;
          iblock = (sector_t)index << (PAGE_CACHE_SHIFT - inode->i_blkbits);
  
          page = grab_cache_page(mapping, index);
          err = -ENOMEM;
          if (!page)
                  goto out;
  
          if (page_has_buffers(page)) {
  has_buffers:
                  unlock_page(page);
                  page_cache_release(page);
                  return block_truncate_page(mapping, from, get_block);
          }
  
          /* Find the buffer that contains "offset" */
          pos = blocksize;
          while (offset >= pos) {
                  iblock++;
                  pos += blocksize;
          }
  
          map_bh.b_size = blocksize;
          map_bh.b_state = 0;
          err = get_block(inode, iblock, &map_bh, 0);
          if (err)
                  goto unlock;
          /* unmapped? It's a hole - nothing to do */
          if (!buffer_mapped(&map_bh))
                  goto unlock;
  
          /* Ok, it's mapped. Make sure it's up-to-date */
          if (!PageUptodate(page)) {
                  err = mapping->a_ops->readpage(NULL, page);
                  if (err) {
                          page_cache_release(page);
                          goto out;
                  }
                  lock_page(page);
                  if (!PageUptodate(page)) {
                          err = -EIO;
                          goto unlock;
                  }
                  if (page_has_buffers(page))
                          goto has_buffers;
          }
          zero_user(page, offset, length);
          set_page_dirty(page);
          err = 0;
  
  unlock:
          unlock_page(page);
          page_cache_release(page);
  out:
          return err;
  }
  EXPORT_SYMBOL(nobh_truncate_page);
  
  int block_truncate_page(struct address_space *mapping,
                          loff_t from, get_block_t *get_block)
  {
          pgoff_t index = from >> PAGE_CACHE_SHIFT;
          unsigned offset = from & (PAGE_CACHE_SIZE-1);
          unsigned blocksize;
          sector_t iblock;
          unsigned length, pos;
          struct inode *inode = mapping->host;
          struct page *page;
          struct buffer_head *bh;
          int err;
  
          blocksize = 1 << inode->i_blkbits;
          length = offset & (blocksize - 1);
  
          /* Block boundary? Nothing to do */
          if (!length)
                  return 0;
  
          length = blocksize - length;
          iblock = (sector_t)index << (PAGE_CACHE_SHIFT - inode->i_blkbits);
          
          page = grab_cache_page(mapping, index);
          err = -ENOMEM;
          if (!page)
                  goto out;
  
          if (!page_has_buffers(page))
                  create_empty_buffers(page, blocksize, 0);
  
          /* Find the buffer that contains "offset" */
          bh = page_buffers(page);
          pos = blocksize;
          while (offset >= pos) {
                  bh = bh->b_this_page;
                  iblock++;
                  pos += blocksize;
          }
  
          err = 0;
          if (!buffer_mapped(bh)) {
                  WARN_ON(bh->b_size != blocksize);
                  err = get_block(inode, iblock, bh, 0);
                  if (err)
                          goto unlock;
                  /* unmapped? It's a hole - nothing to do */
                  if (!buffer_mapped(bh))
                          goto unlock;
          }
  
          /* Ok, it's mapped. Make sure it's up-to-date */
          if (PageUptodate(page))
                  set_buffer_uptodate(bh);
  
          if (!buffer_uptodate(bh) && !buffer_delay(bh) && !buffer_unwritten(bh)) {
                  err = -EIO;
                  ll_rw_block(READ, 1, &bh);
                  wait_on_buffer(bh);
                  /* Uhhuh. Read error. Complain and punt. */
                  if (!buffer_uptodate(bh))
                          goto unlock;
          }
  
          zero_user(page, offset, length);
          mark_buffer_dirty(bh);
          err = 0;
  
  unlock:
          unlock_page(page);
          page_cache_release(page);
  out:
          return err;
  }
  EXPORT_SYMBOL(block_truncate_page);
  
  /*
   * The generic ->writepage function for buffer-backed address_spaces
   * this form passes in the end_io handler used to finish the IO.
   */
  int block_write_full_page_endio(struct page *page, get_block_t *get_block,
                          struct writeback_control *wbc, bh_end_io_t *handler)
  {
          struct inode * const inode = page->mapping->host;
          loff_t i_size = i_size_read(inode);
          const pgoff_t end_index = i_size >> PAGE_CACHE_SHIFT;
          unsigned offset;
  
          /* Is the page fully inside i_size? */
          if (page->index < end_index)
                  return __block_write_full_page(inode, page, get_block, wbc,
                                                 handler);
  
          /* Is the page fully outside i_size? (truncate in progress) */
          offset = i_size & (PAGE_CACHE_SIZE-1);
          if (page->index >= end_index+1 || !offset) {
                  /*
                   * The page may have dirty, unmapped buffers.  For example,
                   * they may have been added in ext3_writepage().  Make them
                   * freeable here, so the page does not leak.
                   */
                  do_invalidatepage(page, 0);
                  unlock_page(page);
                  return 0; /* don't care */
          }
  
          /*
           * The page straddles i_size.  It must be zeroed out on each and every
           * writepage invocation because it may be mmapped.  "A file is mapped
           * in multiples of the page size.  For a file that is not a multiple of
           * the  page size, the remaining memory is zeroed when mapped, and
           * writes to that region are not written out to the file."
           */
          zero_user_segment(page, offset, PAGE_CACHE_SIZE);
          return __block_write_full_page(inode, page, get_block, wbc, handler);
  }
  EXPORT_SYMBOL(block_write_full_page_endio);
  
  /*
   * The generic ->writepage function for buffer-backed address_spaces
   */
  int block_write_full_page(struct page *page, get_block_t *get_block,
                          struct writeback_control *wbc)
  {
          return block_write_full_page_endio(page, get_block, wbc,
                                             end_buffer_async_write);
  }
  EXPORT_SYMBOL(block_write_full_page);
  
  sector_t generic_block_bmap(struct address_space *mapping, sector_t block,
                              get_block_t *get_block)
  {
          struct buffer_head tmp;
          struct inode *inode = mapping->host;
          tmp.b_state = 0;
          tmp.b_blocknr = 0;
          tmp.b_size = 1 << inode->i_blkbits;
          get_block(inode, block, &tmp, 0);
          return tmp.b_blocknr;
  }
  EXPORT_SYMBOL(generic_block_bmap);
  
  static void end_bio_bh_io_sync(struct bio *bio, int err)
  {
          struct buffer_head *bh = bio->bi_private;
  
          if (err == -EOPNOTSUPP) {
                  set_bit(BIO_EOPNOTSUPP, &bio->bi_flags);
          }
  
          if (unlikely (test_bit(BIO_QUIET,&bio->bi_flags)))
                  set_bit(BH_Quiet, &bh->b_state);
  
          bh->b_end_io(bh, test_bit(BIO_UPTODATE, &bio->bi_flags));
          bio_put(bio);
  }
  
  /*
   * This allows us to do IO even on the odd last sectors
   * of a device, even if the bh block size is some multiple
   * of the physical sector size.
   *
   * We'll just truncate the bio to the size of the device,
   * and clear the end of the buffer head manually.
   *
   * Truly out-of-range accesses will turn into actual IO
   * errors, this only handles the "we need to be able to
   * do IO at the final sector" case.
   */
  static void guard_bh_eod(int rw, struct bio *bio, struct buffer_head *bh)
  {
          sector_t maxsector;
          unsigned bytes;
  
          maxsector = i_size_read(bio->bi_bdev->bd_inode) >> 9;
          if (!maxsector)
                  return;
  
          /*
           * If the *whole* IO is past the end of the device,
           * let it through, and the IO layer will turn it into
           * an EIO.
           */
          if (unlikely(bio->bi_sector >= maxsector))
                  return;
  
          maxsector -= bio->bi_sector;
          bytes = bio->bi_size;
          if (likely((bytes >> 9) <= maxsector))
                  return;
  
          /* Uhhuh. We've got a bh that straddles the device size! */
          bytes = maxsector << 9;
  
          /* Truncate the bio.. */
          bio->bi_size = bytes;
          bio->bi_io_vec[0].bv_len = bytes;
  
          /* ..and clear the end of the buffer for reads */
          if ((rw & RW_MASK) == READ) {
                  void *kaddr = kmap_atomic(bh->b_page);
                  memset(kaddr + bh_offset(bh) + bytes, 0, bh->b_size - bytes);
                  kunmap_atomic(kaddr);
                  flush_dcache_page(bh->b_page);
          }
  }
  
  int submit_bh(int rw, struct buffer_head * bh)
  {
          struct bio *bio;
          int ret = 0;
  
          BUG_ON(!buffer_locked(bh));
          BUG_ON(!buffer_mapped(bh));
          BUG_ON(!bh->b_end_io);
          BUG_ON(buffer_delay(bh));
          BUG_ON(buffer_unwritten(bh));
  
          /*
           * Only clear out a write error when rewriting
           */
          if (test_set_buffer_req(bh) && (rw & WRITE))
                  clear_buffer_write_io_error(bh);
  
          /*
           * from here on down, it's all bio -- do the initial mapping,
           * submit_bio -> generic_make_request may further map this bio around
           */
          bio = bio_alloc(GFP_NOIO, 1);
  
          bio->bi_sector = bh->b_blocknr * (bh->b_size >> 9);
          bio->bi_bdev = bh->b_bdev;
          bio->bi_io_vec[0].bv_page = bh->b_page;
          bio->bi_io_vec[0].bv_len = bh->b_size;
          bio->bi_io_vec[0].bv_offset = bh_offset(bh);
  
          bio->bi_vcnt = 1;
          bio->bi_idx = 0;
          bio->bi_size = bh->b_size;
  
          bio->bi_end_io = end_bio_bh_io_sync;
          bio->bi_private = bh;
  
          /* Take care of bh's that straddle the end of the device */
          guard_bh_eod(rw, bio, bh);
  
          bio_get(bio);
          submit_bio(rw, bio);
  
          if (bio_flagged(bio, BIO_EOPNOTSUPP))
                  ret = -EOPNOTSUPP;
  
          bio_put(bio);
          return ret;
  }
  EXPORT_SYMBOL(submit_bh);
  
  /**
   * ll_rw_block: low-level access to block devices (DEPRECATED)
   * @rw: whether to %READ or %WRITE or maybe %READA (readahead)
   * @nr: number of &struct buffer_heads in the array
   * @bhs: array of pointers to &struct buffer_head
   *
   * ll_rw_block() takes an array of pointers to &struct buffer_heads, and
   * requests an I/O operation on them, either a %READ or a %WRITE.  The third
   * %READA option is described in the documentation for generic_make_request()
   * which ll_rw_block() calls.
   *
   * This function drops any buffer that it cannot get a lock on (with the
   * BH_Lock state bit), any buffer that appears to be clean when doing a write
   * request, and any buffer that appears to be up-to-date when doing read
   * request.  Further it marks as clean buffers that are processed for
   * writing (the buffer cache won't assume that they are actually clean
   * until the buffer gets unlocked).
   *
   * ll_rw_block sets b_end_io to simple completion handler that marks
   * the buffer up-to-date (if approriate), unlocks the buffer and wakes
   * any waiters. 
   *
   * All of the buffers must be for the same device, and must also be a
   * multiple of the current approved size for the device.
   */
  void ll_rw_block(int rw, int nr, struct buffer_head *bhs[])
  {
          int i;
  
          for (i = 0; i < nr; i++) {
                  struct buffer_head *bh = bhs[i];
  
                  if (!trylock_buffer(bh))
                          continue;
                  if (rw == WRITE) {
                          if (test_clear_buffer_dirty(bh)) {
                                  bh->b_end_io = end_buffer_write_sync;
                                  get_bh(bh);
                                  submit_bh(WRITE, bh);
                                  continue;
                          }
                  } else {
                          if (!buffer_uptodate(bh)) {
                                  bh->b_end_io = end_buffer_read_sync;
                                  get_bh(bh);
                                  submit_bh(rw, bh);
                                  continue;
                          }
                  }
                  unlock_buffer(bh);
          }
  }
  EXPORT_SYMBOL(ll_rw_block);
  
  void write_dirty_buffer(struct buffer_head *bh, int rw)
  {
          lock_buffer(bh);
          if (!test_clear_buffer_dirty(bh)) {
                  unlock_buffer(bh);
                  return;
          }
          bh->b_end_io = end_buffer_write_sync;
          get_bh(bh);
          submit_bh(rw, bh);
  }
  EXPORT_SYMBOL(write_dirty_buffer);
  
  /*
   * For a data-integrity writeout, we need to wait upon any in-progress I/O
   * and then start new I/O and then wait upon it.  The caller must have a ref on
   * the buffer_head.
   */
  int __sync_dirty_buffer(struct buffer_head *bh, int rw)
  {
          int ret = 0;
  
          WARN_ON(atomic_read(&bh->b_count) < 1);
          lock_buffer(bh);
          if (test_clear_buffer_dirty(bh)) {
                  get_bh(bh);
                  bh->b_end_io = end_buffer_write_sync;
                  ret = submit_bh(rw, bh);
                  wait_on_buffer(bh);
                  if (!ret && !buffer_uptodate(bh))
                          ret = -EIO;
          } else {
                  unlock_buffer(bh);
          }
          return ret;
  }
  EXPORT_SYMBOL(__sync_dirty_buffer);
  
  int sync_dirty_buffer(struct buffer_head *bh)
  {
          return __sync_dirty_buffer(bh, WRITE_SYNC);
  }
  EXPORT_SYMBOL(sync_dirty_buffer);
  
  /*
   * try_to_free_buffers() checks if all the buffers on this particular page
   * are unused, and releases them if so.
   *
   * Exclusion against try_to_free_buffers may be obtained by either
   * locking the page or by holding its mapping's private_lock.
   *
   * If the page is dirty but all the buffers are clean then we need to
   * be sure to mark the page clean as well.  This is because the page
   * may be against a block device, and a later reattachment of buffers
   * to a dirty page will set *all* buffers dirty.  Which would corrupt
   * filesystem data on the same device.
   *
   * The same applies to regular filesystem pages: if all the buffers are
   * clean then we set the page clean and proceed.  To do that, we require
   * total exclusion from __set_page_dirty_buffers().  That is obtained with
   * private_lock.
   *
   * try_to_free_buffers() is non-blocking.
   */
  static inline int buffer_busy(struct buffer_head *bh)
  {
          return atomic_read(&bh->b_count) |
                  (bh->b_state & ((1 << BH_Dirty) | (1 << BH_Lock)));
  }
  
  static int
  drop_buffers(struct page *page, struct buffer_head **buffers_to_free)
  {
          struct buffer_head *head = page_buffers(page);
          struct buffer_head *bh;
  
          bh = head;
          do {
                  if (buffer_write_io_error(bh) && page->mapping)
                          set_bit(AS_EIO, &page->mapping->flags);
                  if (buffer_busy(bh))
                          goto failed;
                  bh = bh->b_this_page;
          } while (bh != head);
  
          do {
                  struct buffer_head *next = bh->b_this_page;
  
                  if (bh->b_assoc_map)
                          __remove_assoc_queue(bh);
                  bh = next;
          } while (bh != head);
          *buffers_to_free = head;
          __clear_page_buffers(page);
          return 1;
  failed:
          return 0;
  }
  
  int try_to_free_buffers(struct page *page)
  {
          struct address_space * const mapping = page->mapping;
          struct buffer_head *buffers_to_free = NULL;
          int ret = 0;
  
          BUG_ON(!PageLocked(page));
          if (PageWriteback(page))
                  return 0;
  
          if (mapping == NULL) {          /* can this still happen? */
                  ret = drop_buffers(page, &buffers_to_free);
                  goto out;
          }
  
          spin_lock(&mapping->private_lock);
          ret = drop_buffers(page, &buffers_to_free);
  
          /*
           * If the filesystem writes its buffers by hand (eg ext3)
           * then we can have clean buffers against a dirty page.  We
           * clean the page here; otherwise the VM will never notice
           * that the filesystem did any IO at all.
           *
           * Also, during truncate, discard_buffer will have marked all
           * the page's buffers clean.  We discover that here and clean
           * the page also.
           *
           * private_lock must be held over this entire operation in order
           * to synchronise against __set_page_dirty_buffers and prevent the
           * dirty bit from being lost.
           */
          if (ret)
                  cancel_dirty_page(page, PAGE_CACHE_SIZE);
          spin_unlock(&mapping->private_lock);
  out:
          if (buffers_to_free) {
                  struct buffer_head *bh = buffers_to_free;
  
                  do {
                          struct buffer_head *next = bh->b_this_page;
                          free_buffer_head(bh);
                          bh = next;
                  } while (bh != buffers_to_free);
          }
          return ret;
  }
  EXPORT_SYMBOL(try_to_free_buffers);
  
  /*
   * There are no bdflush tunables left.  But distributions are
   * still running obsolete flush daemons, so we terminate them here.
   *
   * Use of bdflush() is deprecated and will be removed in a future kernel.
   * The `flush-X' kernel threads fully replace bdflush daemons and this call.
   */
  SYSCALL_DEFINE2(bdflush, int, func, long, data)
  {
          static int msg_count;
  
          if (!capable(CAP_SYS_ADMIN))
                  return -EPERM;
  
          if (msg_count < 5) {
                  msg_count++;
                  printk(KERN_INFO
                          "warning: process `%s' used the obsolete bdflush"
                          " system call\n", current->comm);
                  printk(KERN_INFO "Fix your initscripts?\n");
          }
  
          if (func == 1)
                  do_exit(0);
          return 0;
  }
  
  /*
   * Buffer-head allocation
   */
  static struct kmem_cache *bh_cachep __read_mostly;
  
  /*
   * Once the number of bh's in the machine exceeds this level, we start
   * stripping them in writeback.
   */
  static int max_buffer_heads;
  
  int buffer_heads_over_limit;
  
  struct bh_accounting {
          int nr;                 /* Number of live bh's */
          int ratelimit;          /* Limit cacheline bouncing */
  };
  
  static DEFINE_PER_CPU(struct bh_accounting, bh_accounting) = {0, 0};
  
  static void recalc_bh_state(void)
  {
          int i;
          int tot = 0;
  
          if (__this_cpu_inc_return(bh_accounting.ratelimit) - 1 < 4096)
                  return;
          __this_cpu_write(bh_accounting.ratelimit, 0);
          for_each_online_cpu(i)
                  tot += per_cpu(bh_accounting, i).nr;
          buffer_heads_over_limit = (tot > max_buffer_heads);
  }
  
  struct buffer_head *alloc_buffer_head(gfp_t gfp_flags)
  {
          struct buffer_head *ret = kmem_cache_zalloc(bh_cachep, gfp_flags);
          if (ret) {
                  INIT_LIST_HEAD(&ret->b_assoc_buffers);
                  preempt_disable();
                  __this_cpu_inc(bh_accounting.nr);
                  recalc_bh_state();
                  preempt_enable();
          }
          return ret;
  }
  EXPORT_SYMBOL(alloc_buffer_head);
  
  void free_buffer_head(struct buffer_head *bh)
  {
          BUG_ON(!list_empty(&bh->b_assoc_buffers));
          kmem_cache_free(bh_cachep, bh);
          preempt_disable();
          __this_cpu_dec(bh_accounting.nr);
          recalc_bh_state();
          preempt_enable();
  }
  EXPORT_SYMBOL(free_buffer_head);
  
  static void buffer_exit_cpu(int cpu)
  {
          int i;
          struct bh_lru *b = &per_cpu(bh_lrus, cpu);
  
          for (i = 0; i < BH_LRU_SIZE; i++) {
                  brelse(b->bhs[i]);
                  b->bhs[i] = NULL;
          }
          this_cpu_add(bh_accounting.nr, per_cpu(bh_accounting, cpu).nr);
          per_cpu(bh_accounting, cpu).nr = 0;
  }
  
  static int buffer_cpu_notify(struct notifier_block *self,
                                unsigned long action, void *hcpu)
  {
          if (action == CPU_DEAD || action == CPU_DEAD_FROZEN)
                  buffer_exit_cpu((unsigned long)hcpu);
          return NOTIFY_OK;
  }
  
  /**
   * bh_uptodate_or_lock - Test whether the buffer is uptodate
   * @bh: struct buffer_head
   *
   * Return true if the buffer is up-to-date and false,
   * with the buffer locked, if not.
   */
  int bh_uptodate_or_lock(struct buffer_head *bh)
  {
          if (!buffer_uptodate(bh)) {
                  lock_buffer(bh);
                  if (!buffer_uptodate(bh))
                          return 0;
                  unlock_buffer(bh);
          }
          return 1;
  }
  EXPORT_SYMBOL(bh_uptodate_or_lock);
  
  /**
   * bh_submit_read - Submit a locked buffer for reading
   * @bh: struct buffer_head
   *
   * Returns zero on success and -EIO on error.
   */
  int bh_submit_read(struct buffer_head *bh)
  {
          BUG_ON(!buffer_locked(bh));
  
          if (buffer_uptodate(bh)) {
                  unlock_buffer(bh);
                  return 0;
          }
  
          get_bh(bh);
          bh->b_end_io = end_buffer_read_sync;
          submit_bh(READ, bh);
          wait_on_buffer(bh);
          if (buffer_uptodate(bh))
                  return 0;
          return -EIO;
  }
  EXPORT_SYMBOL(bh_submit_read);
  
  void __init buffer_init(void)
  {
          int nrpages;
  
          bh_cachep = kmem_cache_create("buffer_head",
                          sizeof(struct buffer_head), 0,
                                  (SLAB_RECLAIM_ACCOUNT|SLAB_PANIC|
                                  SLAB_MEM_SPREAD),
                                  NULL);
  
          /*
           * Limit the bh occupancy to 10% of ZONE_NORMAL
           */
          nrpages = (nr_free_buffer_pages() * 10) / 100;
          max_buffer_heads = nrpages * (PAGE_SIZE / sizeof(struct buffer_head));
          hotcpu_notifier(buffer_cpu_notify, 0);
  }