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 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * mm/readahead.c - address_space-level file readahead.
  *
  * Copyright (C) 2002, Linus Torvalds
  *
  * 09Apr2002    Andrew Morton
  *      Initial version.
  */
 
 /**
  * DOC: Readahead Overview
  *
  * Readahead is used to read content into the page cache before it is
  * explicitly requested by the application.  Readahead only ever
  * attempts to read folios that are not yet in the page cache.  If a
  * folio is present but not up-to-date, readahead will not try to read
  * it. In that case a simple ->read_folio() will be requested.
  *
  * Readahead is triggered when an application read request (whether a
  * system call or a page fault) finds that the requested folio is not in
  * the page cache, or that it is in the page cache and has the
  * readahead flag set.  This flag indicates that the folio was read
  * as part of a previous readahead request and now that it has been
  * accessed, it is time for the next readahead.
  *
  * Each readahead request is partly synchronous read, and partly async
  * readahead.  This is reflected in the struct file_ra_state which
  * contains ->size being the total number of pages, and ->async_size
  * which is the number of pages in the async section.  The readahead
  * flag will be set on the first folio in this async section to trigger
  * a subsequent readahead.  Once a series of sequential reads has been
  * established, there should be no need for a synchronous component and
  * all readahead request will be fully asynchronous.
  *
  * When either of the triggers causes a readahead, three numbers need
  * to be determined: the start of the region to read, the size of the
  * region, and the size of the async tail.
  *
  * The start of the region is simply the first page address at or after
  * the accessed address, which is not currently populated in the page
  * cache.  This is found with a simple search in the page cache.
  *
  * The size of the async tail is determined by subtracting the size that
  * was explicitly requested from the determined request size, unless
  * this would be less than zero - then zero is used.  NOTE THIS
  * CALCULATION IS WRONG WHEN THE START OF THE REGION IS NOT THE ACCESSED
  * PAGE.  ALSO THIS CALCULATION IS NOT USED CONSISTENTLY.
  *
  * The size of the region is normally determined from the size of the
  * previous readahead which loaded the preceding pages.  This may be
  * discovered from the struct file_ra_state for simple sequential reads,
  * or from examining the state of the page cache when multiple
  * sequential reads are interleaved.  Specifically: where the readahead
  * was triggered by the readahead flag, the size of the previous
  * readahead is assumed to be the number of pages from the triggering
  * page to the start of the new readahead.  In these cases, the size of
  * the previous readahead is scaled, often doubled, for the new
  * readahead, though see get_next_ra_size() for details.
  *
  * If the size of the previous read cannot be determined, the number of
  * preceding pages in the page cache is used to estimate the size of
  * a previous read.  This estimate could easily be misled by random
  * reads being coincidentally adjacent, so it is ignored unless it is
  * larger than the current request, and it is not scaled up, unless it
  * is at the start of file.
  *
  * In general readahead is accelerated at the start of the file, as
  * reads from there are often sequential.  There are other minor
  * adjustments to the readahead size in various special cases and these
  * are best discovered by reading the code.
  *
  * The above calculation, based on the previous readahead size,
  * determines the size of the readahead, to which any requested read
  * size may be added.
  *
  * Readahead requests are sent to the filesystem using the ->readahead()
  * address space operation, for which mpage_readahead() is a canonical
  * implementation.  ->readahead() should normally initiate reads on all
  * folios, but may fail to read any or all folios without causing an I/O
  * error.  The page cache reading code will issue a ->read_folio() request
  * for any folio which ->readahead() did not read, and only an error
  * from this will be final.
  *
  * ->readahead() will generally call readahead_folio() repeatedly to get
  * each folio from those prepared for readahead.  It may fail to read a
  * folio by:
  *
  * * not calling readahead_folio() sufficiently many times, effectively
  *   ignoring some folios, as might be appropriate if the path to
  *   storage is congested.
  *
  * * failing to actually submit a read request for a given folio,
  *   possibly due to insufficient resources, or
  *
  * * getting an error during subsequent processing of a request.
  *
  * In the last two cases, the folio should be unlocked by the filesystem
  * to indicate that the read attempt has failed.  In the first case the
  * folio will be unlocked by the VFS.
  *
  * Those folios not in the final ``async_size`` of the request should be
  * considered to be important and ->readahead() should not fail them due
  * to congestion or temporary resource unavailability, but should wait
  * for necessary resources (e.g.  memory or indexing information) to
  * become available.  Folios in the final ``async_size`` may be
  * considered less urgent and failure to read them is more acceptable.
  * In this case it is best to use filemap_remove_folio() to remove the
  * folios from the page cache as is automatically done for folios that
  * were not fetched with readahead_folio().  This will allow a
  * subsequent synchronous readahead request to try them again.  If they
  * are left in the page cache, then they will be read individually using
  * ->read_folio() which may be less efficient.
  */
 
 #include <linux/blkdev.h>
 #include <linux/kernel.h>
 #include <linux/dax.h>
 #include <linux/gfp.h>
 #include <linux/export.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>
 #include <linux/mm_inline.h>
 #include <linux/blk-cgroup.h>
 #include <linux/fadvise.h>
 #include <linux/sched/mm.h>
 
 #include "internal.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 = inode_to_bdi(mapping->host)->ra_pages;
     ra->prev_pos = -1;
 }
 EXPORT_SYMBOL_GPL(file_ra_state_init);
 
 static void read_pages(struct readahead_control *rac)
 {
     const struct address_space_operations *aops = rac->mapping->a_ops;
     struct folio *folio;
     struct blk_plug plug;
 
     if (!readahead_count(rac))
         return;
 
     blk_start_plug(&plug);
 
     if (aops->readahead) {
         aops->readahead(rac);
         /*
          * Clean up the remaining folios.  The sizes in ->ra
          * may be used to size the next readahead, so make sure
          * they accurately reflect what happened.
          */
         while ((folio = readahead_folio(rac)) != NULL) {
             unsigned long nr = folio_nr_pages(folio);
 
             folio_get(folio);
             rac->ra->size -= nr;
             if (rac->ra->async_size >= nr) {
                 rac->ra->async_size -= nr;
                 filemap_remove_folio(folio);
             }
             folio_unlock(folio);
             folio_put(folio);
         }
     } else {
         while ((folio = readahead_folio(rac)) != NULL)
             aops->read_folio(rac->file, folio);
     }
 
     blk_finish_plug(&plug);
 
     BUG_ON(readahead_count(rac));
 }
 
 /**
  * page_cache_ra_unbounded - Start unchecked readahead.
  * @ractl: Readahead control.
  * @nr_to_read: The number of pages to read.
  * @lookahead_size: Where to start the next readahead.
  *
  * This function is for filesystems to call when they want to start
  * readahead beyond a file's stated i_size.  This is almost certainly
  * not the function you want to call.  Use page_cache_async_readahead()
  * or page_cache_sync_readahead() instead.
  *
  * Context: File is referenced by caller.  Mutexes may be held by caller.
  * May sleep, but will not reenter filesystem to reclaim memory.
  */
 void page_cache_ra_unbounded(struct readahead_control *ractl,
         unsigned long nr_to_read, unsigned long lookahead_size)
 {
     struct address_space *mapping = ractl->mapping;
     unsigned long index = readahead_index(ractl);
     gfp_t gfp_mask = readahead_gfp_mask(mapping);
     unsigned long i;
 
     /*
      * Partway through the readahead operation, we will have added
      * locked pages to the page cache, but will not yet have submitted
      * them for I/O.  Adding another page may need to allocate memory,
      * which can trigger memory reclaim.  Telling the VM we're in
      * the middle of a filesystem operation will cause it to not
      * touch file-backed pages, preventing a deadlock.  Most (all?)
      * filesystems already specify __GFP_NOFS in their mapping's
      * gfp_mask, but let's be explicit here.
      */
     unsigned int nofs = memalloc_nofs_save();
 
     filemap_invalidate_lock_shared(mapping);
     /*
      * Preallocate as many pages as we will need.
      */
     for (i = 0; i < nr_to_read; i++) {
         struct folio *folio = xa_load(&mapping->i_pages, index + i);
 
         if (folio && !xa_is_value(folio)) {
             /*
              * Page already present?  Kick off the current batch
              * of contiguous pages before continuing with the
              * next batch.  This page may be the one we would
              * have intended to mark as Readahead, but we don't
              * have a stable reference to this page, and it's
              * not worth getting one just for that.
              */
             read_pages(ractl);
             ractl->_index++;
             i = ractl->_index + ractl->_nr_pages - index - 1;
             continue;
         }
 
         folio = filemap_alloc_folio(gfp_mask, 0);
         if (!folio)
             break;
         if (filemap_add_folio(mapping, folio, index + i,
                     gfp_mask) < 0) {
             folio_put(folio);
             read_pages(ractl);
             ractl->_index++;
             i = ractl->_index + ractl->_nr_pages - index - 1;
             continue;
         }
         if (i == nr_to_read - lookahead_size)
             folio_set_readahead(folio);
         ractl->_nr_pages++;
     }
 
     /*
      * Now start the IO.  We ignore I/O errors - if the folio is not
      * uptodate then the caller will launch read_folio again, and
      * will then handle the error.
      */
     read_pages(ractl);
     filemap_invalidate_unlock_shared(mapping);
     memalloc_nofs_restore(nofs);
 }
 EXPORT_SYMBOL_GPL(page_cache_ra_unbounded);
 
 /*
  * do_page_cache_ra() actually reads a chunk of disk.  It allocates
  * the pages first, then submits them 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.
  */
 static void do_page_cache_ra(struct readahead_control *ractl,
         unsigned long nr_to_read, unsigned long lookahead_size)
 {
     struct inode *inode = ractl->mapping->host;
     unsigned long index = readahead_index(ractl);
     loff_t isize = i_size_read(inode);
     pgoff_t end_index;  /* The last page we want to read */
 
     if (isize == 0)
         return;
 
     end_index = (isize - 1) >> PAGE_SHIFT;
     if (index > end_index)
         return;
     /* Don't read past the page containing the last byte of the file */
     if (nr_to_read > end_index - index)
         nr_to_read = end_index - index + 1;
 
     page_cache_ra_unbounded(ractl, nr_to_read, lookahead_size);
 }
 
 /*
  * Chunk the readahead into 2 megabyte units, so that we don't pin too much
  * memory at once.
  */
 void force_page_cache_ra(struct readahead_control *ractl,
         unsigned long nr_to_read)
 {
     struct address_space *mapping = ractl->mapping;
     struct file_ra_state *ra = ractl->ra;
     struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
     unsigned long max_pages, index;
 
     if (unlikely(!mapping->a_ops->read_folio && !mapping->a_ops->readahead))
         return;
 
     /*
      * If the request exceeds the readahead window, allow the read to
      * be up to the optimal hardware IO size
      */
     index = readahead_index(ractl);
     max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
     nr_to_read = min_t(unsigned long, nr_to_read, max_pages);
     while (nr_to_read) {
         unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
 
         if (this_chunk > nr_to_read)
             this_chunk = nr_to_read;
         ractl->_index = index;
         do_page_cache_ra(ractl, this_chunk, 0);
 
         index += this_chunk;
         nr_to_read -= this_chunk;
     }
 }
 
 /*
  * 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
  * 1-2 page = 16k, 3-4 page 32k, 5-8 page = 64k, > 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;
 
     if (cur < max / 16)
         return 4 * cur;
     if (cur <= max / 2)
         return 2 * cur;
     return 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 @index-1 to @index-@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,
                    pgoff_t index, unsigned long max)
 {
     pgoff_t head;
 
     rcu_read_lock();
     head = page_cache_prev_miss(mapping, index - 1, max);
     rcu_read_unlock();
 
     return index - 1 - head;
 }
 
 /*
  * page cache context based readahead
  */
 static int try_context_readahead(struct address_space *mapping,
                  struct file_ra_state *ra,
                  pgoff_t index,
                  unsigned long req_size,
                  unsigned long max)
 {
     pgoff_t size;
 
     size = count_history_pages(mapping, index, max);
 
     /*
      * not enough history pages:
      * it could be a random read
      */
     if (size <= req_size)
         return 0;
 
     /*
      * starts from beginning of file:
      * it is a strong indication of long-run stream (or whole-file-read)
      */
     if (size >= index)
         size *= 2;
 
     ra->start = index;
     ra->size = min(size + req_size, max);
     ra->async_size = 1;
 
     return 1;
 }
 
 /*
  * There are some parts of the kernel which assume that PMD entries
  * are exactly HPAGE_PMD_ORDER.  Those should be fixed, but until then,
  * limit the maximum allocation order to PMD size.  I'm not aware of any
  * assumptions about maximum order if THP are disabled, but 8 seems like
  * a good order (that's 1MB if you're using 4kB pages)
  */
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 #define MAX_PAGECACHE_ORDER HPAGE_PMD_ORDER
 #else
 #define MAX_PAGECACHE_ORDER 8
 #endif
 
 static inline int ra_alloc_folio(struct readahead_control *ractl, pgoff_t index,
         pgoff_t mark, unsigned int order, gfp_t gfp)
 {
     int err;
     struct folio *folio = filemap_alloc_folio(gfp, order);
 
     if (!folio)
         return -ENOMEM;
     mark = round_up(mark, 1UL << order);
     if (index == mark)
         folio_set_readahead(folio);
     err = filemap_add_folio(ractl->mapping, folio, index, gfp);
     if (err)
         folio_put(folio);
     else
         ractl->_nr_pages += 1UL << order;
     return err;
 }
 
 void page_cache_ra_order(struct readahead_control *ractl,
         struct file_ra_state *ra, unsigned int new_order)
 {
     struct address_space *mapping = ractl->mapping;
     pgoff_t index = readahead_index(ractl);
     pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
     pgoff_t mark = index + ra->size - ra->async_size;
     int err = 0;
     gfp_t gfp = readahead_gfp_mask(mapping);
 
     if (!mapping_large_folio_support(mapping) || ra->size < 4)
         goto fallback;
 
     limit = min(limit, index + ra->size - 1);
 
     if (new_order < MAX_PAGECACHE_ORDER) {
         new_order += 2;
         if (new_order > MAX_PAGECACHE_ORDER)
             new_order = MAX_PAGECACHE_ORDER;
         while ((1 << new_order) > ra->size)
             new_order--;
     }
 
     filemap_invalidate_lock_shared(mapping);
     while (index <= limit) {
         unsigned int order = new_order;
 
         /* Align with smaller pages if needed */
         if (index & ((1UL << order) - 1)) {
             order = __ffs(index);
             if (order == 1)
                 order = 0;
         }
         /* Don't allocate pages past EOF */
         while (index + (1UL << order) - 1 > limit) {
             if (--order == 1)
                 order = 0;
         }
         err = ra_alloc_folio(ractl, index, mark, order, gfp);
         if (err)
             break;
         index += 1UL << order;
     }
 
     if (index > limit) {
         ra->size += index - limit - 1;
         ra->async_size += index - limit - 1;
     }
 
     read_pages(ractl);
     filemap_invalidate_unlock_shared(mapping);
 
     /*
      * If there were already pages in the page cache, then we may have
      * left some gaps.  Let the regular readahead code take care of this
      * situation.
      */
     if (!err)
         return;
 fallback:
     do_page_cache_ra(ractl, ra->size, ra->async_size);
 }
 
 /*
  * A minimal readahead algorithm for trivial sequential/random reads.
  */
 static void ondemand_readahead(struct readahead_control *ractl,
         struct folio *folio, unsigned long req_size)
 {
     struct backing_dev_info *bdi = inode_to_bdi(ractl->mapping->host);
     struct file_ra_state *ra = ractl->ra;
     unsigned long max_pages = ra->ra_pages;
     unsigned long add_pages;
     pgoff_t index = readahead_index(ractl);
     pgoff_t expected, prev_index;
     unsigned int order = folio ? folio_order(folio) : 0;
 
     /*
      * If the request exceeds the readahead window, allow the read to
      * be up to the optimal hardware IO size
      */
     if (req_size > max_pages && bdi->io_pages > max_pages)
         max_pages = min(req_size, bdi->io_pages);
 
     /*
      * start of file
      */
     if (!index)
         goto initial_readahead;
 
     /*
      * It's the expected callback index, assume sequential access.
      * Ramp up sizes, and push forward the readahead window.
      */
     expected = round_up(ra->start + ra->size - ra->async_size,
             1UL << order);
     if (index == expected || index == (ra->start + ra->size)) {
         ra->start += ra->size;
         ra->size = get_next_ra_size(ra, max_pages);
         ra->async_size = ra->size;
         goto readit;
     }
 
     /*
      * Hit a marked folio 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 (folio) {
         pgoff_t start;
 
         rcu_read_lock();
         start = page_cache_next_miss(ractl->mapping, index + 1,
                 max_pages);
         rcu_read_unlock();
 
         if (!start || start - index > max_pages)
             return;
 
         ra->start = start;
         ra->size = start - index;   /* old async_size */
         ra->size += req_size;
         ra->size = get_next_ra_size(ra, max_pages);
         ra->async_size = ra->size;
         goto readit;
     }
 
     /*
      * oversize read
      */
     if (req_size > max_pages)
         goto initial_readahead;
 
     /*
      * sequential cache miss
      * trivial case: (index - prev_index) == 1
      * unaligned reads: (index - prev_index) == 0
      */
     prev_index = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
     if (index - prev_index <= 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(ractl->mapping, ra, index, req_size,
             max_pages))
         goto readit;
 
     /*
      * standalone, small random read
      * Read as is, and do not pollute the readahead state.
      */
     do_page_cache_ra(ractl, req_size, 0);
     return;
 
 initial_readahead:
     ra->start = index;
     ra->size = get_init_ra_size(req_size, max_pages);
     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.
      * Take care of maximum IO pages as above.
      */
     if (index == ra->start && ra->size == ra->async_size) {
         add_pages = get_next_ra_size(ra, max_pages);
         if (ra->size + add_pages <= max_pages) {
             ra->async_size = add_pages;
             ra->size += add_pages;
         } else {
             ra->size = max_pages;
             ra->async_size = max_pages >> 1;
         }
     }
 
     ractl->_index = ra->start;
     page_cache_ra_order(ractl, ra, order);
 }
 
 void page_cache_sync_ra(struct readahead_control *ractl,
         unsigned long req_count)
 {
     bool do_forced_ra = ractl->file && (ractl->file->f_mode & FMODE_RANDOM);
 
     /*
      * Even if readahead is disabled, issue this request as readahead
      * as we'll need it to satisfy the requested range. The forced
      * readahead will do the right thing and limit the read to just the
      * requested range, which we'll set to 1 page for this case.
      */
     if (!ractl->ra->ra_pages || blk_cgroup_congested()) {
         if (!ractl->file)
             return;
         req_count = 1;
         do_forced_ra = true;
     }
 
     /* be dumb */
     if (do_forced_ra) {
         force_page_cache_ra(ractl, req_count);
         return;
     }
 
     ondemand_readahead(ractl, NULL, req_count);
 }
 EXPORT_SYMBOL_GPL(page_cache_sync_ra);
 
 void page_cache_async_ra(struct readahead_control *ractl,
         struct folio *folio, unsigned long req_count)
 {
     /* no readahead */
     if (!ractl->ra->ra_pages)
         return;
 
     /*
      * Same bit is used for PG_readahead and PG_reclaim.
      */
     if (folio_test_writeback(folio))
         return;
 
     folio_clear_readahead(folio);
 
     if (blk_cgroup_congested())
         return;
 
     ondemand_readahead(ractl, folio, req_count);
 }
 EXPORT_SYMBOL_GPL(page_cache_async_ra);
 
 ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
 {
     ssize_t ret;
     struct fd f;
 
     ret = -EBADF;
     f = fdget(fd);
     if (!f.file || !(f.file->f_mode & FMODE_READ))
         goto out;
 
     /*
      * The readahead() syscall is intended to run only on files
      * that can execute readahead. If readahead is not possible
      * on this file, then we must return -EINVAL.
      */
     ret = -EINVAL;
     if (!f.file->f_mapping || !f.file->f_mapping->a_ops ||
         !S_ISREG(file_inode(f.file)->i_mode))
         goto out;
 
     ret = vfs_fadvise(f.file, offset, count, POSIX_FADV_WILLNEED);
 out:
     fdput(f);
     return ret;
 }
 
 SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
 {
     return ksys_readahead(fd, offset, count);
 }
 
 #if defined(CONFIG_COMPAT) && defined(__ARCH_WANT_COMPAT_READAHEAD)
 COMPAT_SYSCALL_DEFINE4(readahead, int, fd, compat_arg_u64_dual(offset), size_t, count)
 {
     return ksys_readahead(fd, compat_arg_u64_glue(offset), count);
 }
 #endif
 
 /**
  * readahead_expand - Expand a readahead request
  * @ractl: The request to be expanded
  * @new_start: The revised start
  * @new_len: The revised size of the request
  *
  * Attempt to expand a readahead request outwards from the current size to the
  * specified size by inserting locked pages before and after the current window
  * to increase the size to the new window.  This may involve the insertion of
  * THPs, in which case the window may get expanded even beyond what was
  * requested.
  *
  * The algorithm will stop if it encounters a conflicting page already in the
  * pagecache and leave a smaller expansion than requested.
  *
  * The caller must check for this by examining the revised @ractl object for a
  * different expansion than was requested.
  */
 void readahead_expand(struct readahead_control *ractl,
               loff_t new_start, size_t new_len)
 {
     struct address_space *mapping = ractl->mapping;
     struct file_ra_state *ra = ractl->ra;
     pgoff_t new_index, new_nr_pages;
     gfp_t gfp_mask = readahead_gfp_mask(mapping);
 
     new_index = new_start / PAGE_SIZE;
 
     /* Expand the leading edge downwards */
     while (ractl->_index > new_index) {
         unsigned long index = ractl->_index - 1;
         struct page *page = xa_load(&mapping->i_pages, index);
 
         if (page && !xa_is_value(page))
             return; /* Page apparently present */
 
         page = __page_cache_alloc(gfp_mask);
         if (!page)
             return;
         if (add_to_page_cache_lru(page, mapping, index, gfp_mask) < 0) {
             put_page(page);
             return;
         }
 
         ractl->_nr_pages++;
         ractl->_index = page->index;
     }
 
     new_len += new_start - readahead_pos(ractl);
     new_nr_pages = DIV_ROUND_UP(new_len, PAGE_SIZE);
 
     /* Expand the trailing edge upwards */
     while (ractl->_nr_pages < new_nr_pages) {
         unsigned long index = ractl->_index + ractl->_nr_pages;
         struct page *page = xa_load(&mapping->i_pages, index);
 
         if (page && !xa_is_value(page))
             return; /* Page apparently present */
 
         page = __page_cache_alloc(gfp_mask);
         if (!page)
             return;
         if (add_to_page_cache_lru(page, mapping, index, gfp_mask) < 0) {
             put_page(page);
             return;
         }
         ractl->_nr_pages++;
         if (ra) {
             ra->size++;
             ra->async_size++;
         }
     }
 }
 EXPORT_SYMBOL(readahead_expand);

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