FreeBSD/Linux Kernel Cross Reference
sys/mm/readahead.c

     /*
      * mm/readahead.c - address_space-level file readahead.
      *
      * Copyright (C) 2002, Linus Torvalds
      *
      * 09Apr2002    Andrew Morton
      *              Initial version.
      */
     
    #include <linux/kernel.h>
    #include <linux/fs.h>
    #include <linux/gfp.h>
    #include <linux/mm.h>
    #include <linux/export.h>
    #include <linux/blkdev.h>
    #include <linux/backing-dev.h>
    #include <linux/task_io_accounting_ops.h>
    #include <linux/pagevec.h>
    #include <linux/pagemap.h>
    #include <linux/syscalls.h>
    #include <linux/file.h>
    
    /*
     * Initialise a struct file's readahead state.  Assumes that the caller has
     * memset *ra to zero.
     */
    void
    file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
    {
            ra->ra_pages = mapping->backing_dev_info->ra_pages;
            ra->prev_pos = -1;
    }
    EXPORT_SYMBOL_GPL(file_ra_state_init);
    
    #define list_to_page(head) (list_entry((head)->prev, struct page, lru))
    
    /*
     * see if a page needs releasing upon read_cache_pages() failure
     * - the caller of read_cache_pages() may have set PG_private or PG_fscache
     *   before calling, such as the NFS fs marking pages that are cached locally
     *   on disk, thus we need to give the fs a chance to clean up in the event of
     *   an error
     */
    static void read_cache_pages_invalidate_page(struct address_space *mapping,
                                                 struct page *page)
    {
            if (page_has_private(page)) {
                    if (!trylock_page(page))
                            BUG();
                    page->mapping = mapping;
                    do_invalidatepage(page, 0);
                    page->mapping = NULL;
                    unlock_page(page);
            }
            page_cache_release(page);
    }
    
    /*
     * release a list of pages, invalidating them first if need be
     */
    static void read_cache_pages_invalidate_pages(struct address_space *mapping,
                                                  struct list_head *pages)
    {
            struct page *victim;
    
            while (!list_empty(pages)) {
                    victim = list_to_page(pages);
                    list_del(&victim->lru);
                    read_cache_pages_invalidate_page(mapping, victim);
            }
    }
    
    /**
     * read_cache_pages - populate an address space with some pages & start reads against them
     * @mapping: the address_space
     * @pages: The address of a list_head which contains the target pages.  These
     *   pages have their ->index populated and are otherwise uninitialised.
     * @filler: callback routine for filling a single page.
     * @data: private data for the callback routine.
     *
     * Hides the details of the LRU cache etc from the filesystems.
     */
    int read_cache_pages(struct address_space *mapping, struct list_head *pages,
                            int (*filler)(void *, struct page *), void *data)
    {
            struct page *page;
            int ret = 0;
    
            while (!list_empty(pages)) {
                    page = list_to_page(pages);
                    list_del(&page->lru);
                    if (add_to_page_cache_lru(page, mapping,
                                            page->index, GFP_KERNEL)) {
                            read_cache_pages_invalidate_page(mapping, page);
                            continue;
                    }
                    page_cache_release(page);
    
                    ret = filler(data, page);
                   if (unlikely(ret)) {
                           read_cache_pages_invalidate_pages(mapping, pages);
                           break;
                   }
                   task_io_account_read(PAGE_CACHE_SIZE);
           }
           return ret;
   }
   
   EXPORT_SYMBOL(read_cache_pages);
   
   static int read_pages(struct address_space *mapping, struct file *filp,
                   struct list_head *pages, unsigned nr_pages)
   {
           struct blk_plug plug;
           unsigned page_idx;
           int ret;
   
           blk_start_plug(&plug);
   
           if (mapping->a_ops->readpages) {
                   ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
                   /* Clean up the remaining pages */
                   put_pages_list(pages);
                   goto out;
           }
   
           for (page_idx = 0; page_idx < nr_pages; page_idx++) {
                   struct page *page = list_to_page(pages);
                   list_del(&page->lru);
                   if (!add_to_page_cache_lru(page, mapping,
                                           page->index, GFP_KERNEL)) {
                           mapping->a_ops->readpage(filp, page);
                   }
                   page_cache_release(page);
           }
           ret = 0;
   
   out:
           blk_finish_plug(&plug);
   
           return ret;
   }
   
   /*
    * __do_page_cache_readahead() actually reads a chunk of disk.  It allocates all
    * the pages first, then submits them all for I/O. This avoids the very bad
    * behaviour which would occur if page allocations are causing VM writeback.
    * We really don't want to intermingle reads and writes like that.
    *
    * Returns the number of pages requested, or the maximum amount of I/O allowed.
    */
   static int
   __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
                           pgoff_t offset, unsigned long nr_to_read,
                           unsigned long lookahead_size)
   {
           struct inode *inode = mapping->host;
           struct page *page;
           unsigned long end_index;        /* The last page we want to read */
           LIST_HEAD(page_pool);
           int page_idx;
           int ret = 0;
           loff_t isize = i_size_read(inode);
   
           if (isize == 0)
                   goto out;
   
           end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
   
           /*
            * Preallocate as many pages as we will need.
            */
           for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
                   pgoff_t page_offset = offset + page_idx;
   
                   if (page_offset > end_index)
                           break;
   
                   rcu_read_lock();
                   page = radix_tree_lookup(&mapping->page_tree, page_offset);
                   rcu_read_unlock();
                   if (page)
                           continue;
   
                   page = page_cache_alloc_readahead(mapping);
                   if (!page)
                           break;
                   page->index = page_offset;
                   list_add(&page->lru, &page_pool);
                   if (page_idx == nr_to_read - lookahead_size)
                           SetPageReadahead(page);
                   ret++;
           }
   
           /*
            * Now start the IO.  We ignore I/O errors - if the page is not
            * uptodate then the caller will launch readpage again, and
            * will then handle the error.
            */
           if (ret)
                   read_pages(mapping, filp, &page_pool, ret);
           BUG_ON(!list_empty(&page_pool));
   out:
           return ret;
   }
   
   /*
    * Chunk the readahead into 2 megabyte units, so that we don't pin too much
    * memory at once.
    */
   int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
                   pgoff_t offset, unsigned long nr_to_read)
   {
           int ret = 0;
   
           if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
                   return -EINVAL;
   
           nr_to_read = max_sane_readahead(nr_to_read);
           while (nr_to_read) {
                   int err;
   
                   unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;
   
                   if (this_chunk > nr_to_read)
                           this_chunk = nr_to_read;
                   err = __do_page_cache_readahead(mapping, filp,
                                                   offset, this_chunk, 0);
                   if (err < 0) {
                           ret = err;
                           break;
                   }
                   ret += err;
                   offset += this_chunk;
                   nr_to_read -= this_chunk;
           }
           return ret;
   }
   
   /*
    * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
    * sensible upper limit.
    */
   unsigned long max_sane_readahead(unsigned long nr)
   {
           return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE)
                   + node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
   }
   
   /*
    * Submit IO for the read-ahead request in file_ra_state.
    */
   unsigned long ra_submit(struct file_ra_state *ra,
                          struct address_space *mapping, struct file *filp)
   {
           int actual;
   
           actual = __do_page_cache_readahead(mapping, filp,
                                           ra->start, ra->size, ra->async_size);
   
           return actual;
   }
   
   /*
    * Set the initial window size, round to next power of 2 and square
    * for small size, x 4 for medium, and x 2 for large
    * for 128k (32 page) max ra
    * 1-8 page = 32k initial, > 8 page = 128k initial
    */
   static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
   {
           unsigned long newsize = roundup_pow_of_two(size);
   
           if (newsize <= max / 32)
                   newsize = newsize * 4;
           else if (newsize <= max / 4)
                   newsize = newsize * 2;
           else
                   newsize = max;
   
           return newsize;
   }
   
   /*
    *  Get the previous window size, ramp it up, and
    *  return it as the new window size.
    */
   static unsigned long get_next_ra_size(struct file_ra_state *ra,
                                                   unsigned long max)
   {
           unsigned long cur = ra->size;
           unsigned long newsize;
   
           if (cur < max / 16)
                   newsize = 4 * cur;
           else
                   newsize = 2 * cur;
   
           return min(newsize, max);
   }
   
   /*
    * On-demand readahead design.
    *
    * The fields in struct file_ra_state represent the most-recently-executed
    * readahead attempt:
    *
    *                        |<----- async_size ---------|
    *     |------------------- size -------------------->|
    *     |==================#===========================|
    *     ^start             ^page marked with PG_readahead
    *
    * To overlap application thinking time and disk I/O time, we do
    * `readahead pipelining': Do not wait until the application consumed all
    * readahead pages and stalled on the missing page at readahead_index;
    * Instead, submit an asynchronous readahead I/O as soon as there are
    * only async_size pages left in the readahead window. Normally async_size
    * will be equal to size, for maximum pipelining.
    *
    * In interleaved sequential reads, concurrent streams on the same fd can
    * be invalidating each other's readahead state. So we flag the new readahead
    * page at (start+size-async_size) with PG_readahead, and use it as readahead
    * indicator. The flag won't be set on already cached pages, to avoid the
    * readahead-for-nothing fuss, saving pointless page cache lookups.
    *
    * prev_pos tracks the last visited byte in the _previous_ read request.
    * It should be maintained by the caller, and will be used for detecting
    * small random reads. Note that the readahead algorithm checks loosely
    * for sequential patterns. Hence interleaved reads might be served as
    * sequential ones.
    *
    * There is a special-case: if the first page which the application tries to
    * read happens to be the first page of the file, it is assumed that a linear
    * read is about to happen and the window is immediately set to the initial size
    * based on I/O request size and the max_readahead.
    *
    * The code ramps up the readahead size aggressively at first, but slow down as
    * it approaches max_readhead.
    */
   
   /*
    * Count contiguously cached pages from @offset-1 to @offset-@max,
    * this count is a conservative estimation of
    *      - length of the sequential read sequence, or
    *      - thrashing threshold in memory tight systems
    */
   static pgoff_t count_history_pages(struct address_space *mapping,
                                      struct file_ra_state *ra,
                                      pgoff_t offset, unsigned long max)
   {
           pgoff_t head;
   
           rcu_read_lock();
           head = radix_tree_prev_hole(&mapping->page_tree, offset - 1, max);
           rcu_read_unlock();
   
           return offset - 1 - head;
   }
   
   /*
    * page cache context based read-ahead
    */
   static int try_context_readahead(struct address_space *mapping,
                                    struct file_ra_state *ra,
                                    pgoff_t offset,
                                    unsigned long req_size,
                                    unsigned long max)
   {
           pgoff_t size;
   
           size = count_history_pages(mapping, ra, offset, max);
   
           /*
            * no history pages:
            * it could be a random read
            */
           if (!size)
                   return 0;
   
           /*
            * starts from beginning of file:
            * it is a strong indication of long-run stream (or whole-file-read)
            */
           if (size >= offset)
                   size *= 2;
   
           ra->start = offset;
           ra->size = get_init_ra_size(size + req_size, max);
           ra->async_size = ra->size;
   
           return 1;
   }
   
   /*
    * A minimal readahead algorithm for trivial sequential/random reads.
    */
   static unsigned long
   ondemand_readahead(struct address_space *mapping,
                      struct file_ra_state *ra, struct file *filp,
                      bool hit_readahead_marker, pgoff_t offset,
                      unsigned long req_size)
   {
           unsigned long max = max_sane_readahead(ra->ra_pages);
   
           /*
            * start of file
            */
           if (!offset)
                   goto initial_readahead;
   
           /*
            * It's the expected callback offset, assume sequential access.
            * Ramp up sizes, and push forward the readahead window.
            */
           if ((offset == (ra->start + ra->size - ra->async_size) ||
                offset == (ra->start + ra->size))) {
                   ra->start += ra->size;
                   ra->size = get_next_ra_size(ra, max);
                   ra->async_size = ra->size;
                   goto readit;
           }
   
           /*
            * Hit a marked page without valid readahead state.
            * E.g. interleaved reads.
            * Query the pagecache for async_size, which normally equals to
            * readahead size. Ramp it up and use it as the new readahead size.
            */
           if (hit_readahead_marker) {
                   pgoff_t start;
   
                   rcu_read_lock();
                   start = radix_tree_next_hole(&mapping->page_tree, offset+1,max);
                   rcu_read_unlock();
   
                   if (!start || start - offset > max)
                           return 0;
   
                   ra->start = start;
                   ra->size = start - offset;      /* old async_size */
                   ra->size += req_size;
                   ra->size = get_next_ra_size(ra, max);
                   ra->async_size = ra->size;
                   goto readit;
           }
   
           /*
            * oversize read
            */
           if (req_size > max)
                   goto initial_readahead;
   
           /*
            * sequential cache miss
            */
           if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
                   goto initial_readahead;
   
           /*
            * Query the page cache and look for the traces(cached history pages)
            * that a sequential stream would leave behind.
            */
           if (try_context_readahead(mapping, ra, offset, req_size, max))
                   goto readit;
   
           /*
            * standalone, small random read
            * Read as is, and do not pollute the readahead state.
            */
           return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
   
   initial_readahead:
           ra->start = offset;
           ra->size = get_init_ra_size(req_size, max);
           ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
   
   readit:
           /*
            * Will this read hit the readahead marker made by itself?
            * If so, trigger the readahead marker hit now, and merge
            * the resulted next readahead window into the current one.
            */
           if (offset == ra->start && ra->size == ra->async_size) {
                   ra->async_size = get_next_ra_size(ra, max);
                   ra->size += ra->async_size;
           }
   
           return ra_submit(ra, mapping, filp);
   }
   
   /**
    * page_cache_sync_readahead - generic file readahead
    * @mapping: address_space which holds the pagecache and I/O vectors
    * @ra: file_ra_state which holds the readahead state
    * @filp: passed on to ->readpage() and ->readpages()
    * @offset: start offset into @mapping, in pagecache page-sized units
    * @req_size: hint: total size of the read which the caller is performing in
    *            pagecache pages
    *
    * page_cache_sync_readahead() should be called when a cache miss happened:
    * it will submit the read.  The readahead logic may decide to piggyback more
    * pages onto the read request if access patterns suggest it will improve
    * performance.
    */
   void page_cache_sync_readahead(struct address_space *mapping,
                                  struct file_ra_state *ra, struct file *filp,
                                  pgoff_t offset, unsigned long req_size)
   {
           /* no read-ahead */
           if (!ra->ra_pages)
                   return;
   
           /* be dumb */
           if (filp && (filp->f_mode & FMODE_RANDOM)) {
                   force_page_cache_readahead(mapping, filp, offset, req_size);
                   return;
           }
   
           /* do read-ahead */
           ondemand_readahead(mapping, ra, filp, false, offset, req_size);
   }
   EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
   
   /**
    * page_cache_async_readahead - file readahead for marked pages
    * @mapping: address_space which holds the pagecache and I/O vectors
    * @ra: file_ra_state which holds the readahead state
    * @filp: passed on to ->readpage() and ->readpages()
    * @page: the page at @offset which has the PG_readahead flag set
    * @offset: start offset into @mapping, in pagecache page-sized units
    * @req_size: hint: total size of the read which the caller is performing in
    *            pagecache pages
    *
    * page_cache_async_readahead() should be called when a page is used which
    * has the PG_readahead flag; this is a marker to suggest that the application
    * has used up enough of the readahead window that we should start pulling in
    * more pages.
    */
   void
   page_cache_async_readahead(struct address_space *mapping,
                              struct file_ra_state *ra, struct file *filp,
                              struct page *page, pgoff_t offset,
                              unsigned long req_size)
   {
           /* no read-ahead */
           if (!ra->ra_pages)
                   return;
   
           /*
            * Same bit is used for PG_readahead and PG_reclaim.
            */
           if (PageWriteback(page))
                   return;
   
           ClearPageReadahead(page);
   
           /*
            * Defer asynchronous read-ahead on IO congestion.
            */
           if (bdi_read_congested(mapping->backing_dev_info))
                   return;
   
           /* do read-ahead */
           ondemand_readahead(mapping, ra, filp, true, offset, req_size);
   }
   EXPORT_SYMBOL_GPL(page_cache_async_readahead);
   
   static ssize_t
   do_readahead(struct address_space *mapping, struct file *filp,
                pgoff_t index, unsigned long nr)
   {
           if (!mapping || !mapping->a_ops || !mapping->a_ops->readpage)
                   return -EINVAL;
   
           force_page_cache_readahead(mapping, filp, index, nr);
           return 0;
   }
   
   SYSCALL_DEFINE(readahead)(int fd, loff_t offset, size_t count)
   {
           ssize_t ret;
           struct fd f;
   
           ret = -EBADF;
           f = fdget(fd);
           if (f.file) {
                   if (f.file->f_mode & FMODE_READ) {
                           struct address_space *mapping = f.file->f_mapping;
                           pgoff_t start = offset >> PAGE_CACHE_SHIFT;
                           pgoff_t end = (offset + count - 1) >> PAGE_CACHE_SHIFT;
                           unsigned long len = end - start + 1;
                           ret = do_readahead(mapping, f.file, start, len);
                   }
                   fdput(f);
           }
           return ret;
   }
   #ifdef CONFIG_HAVE_SYSCALL_WRAPPERS
   asmlinkage long SyS_readahead(long fd, loff_t offset, long count)
   {
           return SYSC_readahead((int) fd, offset, (size_t) count);
   }
   SYSCALL_ALIAS(sys_readahead, SyS_readahead);
   #endif

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