Quelle readahead.c Sprache: C

// 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/pagemap.h>
#include <linux/psi.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;

if (unlikely(rac->_workingset))
  psi_memstall_enter(&rac->_pflags);
blk_start_plug(&plug);

if (aops->readahead) {
  aops->readahead(rac);
  /* Clean up the remaining folios. */
  while ((folio = readahead_folio(rac)) != NULL) {
   folio_get(folio);
   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);
if (unlikely(rac->_workingset))
  psi_memstall_leave(&rac->_pflags);
rac->_workingset = false;

BUG_ON(readahead_count(rac));
}

static struct folio *ractl_alloc_folio(struct readahead_control *ractl,
           gfp_t gfp_mask, unsigned int order)
{
struct folio *folio;

folio = filemap_alloc_folio(gfp_mask, order);
if (folio && ractl->dropbehind)
  __folio_set_dropbehind(folio);

return folio;
}

/**
* 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 mark = ULONG_MAX, i = 0;
unsigned int min_nrpages = mapping_min_folio_nrpages(mapping);

/*
* 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);
index = mapping_align_index(mapping, index);

/*
* As iterator `i` is aligned to min_nrpages, round_up the
* difference between nr_to_read and lookahead_size to mark the
* index that only has lookahead or "async_region" to set the
* readahead flag.
*/
if (lookahead_size <= nr_to_read) {
  unsigned long ra_folio_index;

  ra_folio_index = round_up(readahead_index(ractl) +
       nr_to_read - lookahead_size,
       min_nrpages);
  mark = ra_folio_index - index;
}
nr_to_read += readahead_index(ractl) - index;
ractl->_index = index;

/*
* Preallocate as many pages as we will need.
*/
while (i < nr_to_read) {
  struct folio *folio = xa_load(&mapping->i_pages, index + i);
  int ret;

  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 += min_nrpages;
   i = ractl->_index + ractl->_nr_pages - index;
   continue;
  }

  folio = ractl_alloc_folio(ractl, gfp_mask,
     mapping_min_folio_order(mapping));
  if (!folio)
   break;

  ret = filemap_add_folio(mapping, folio, index + i, gfp_mask);
  if (ret < 0) {
   folio_put(folio);
   if (ret == -ENOMEM)
    break;
   read_pages(ractl);
   ractl->_index += min_nrpages;
   i = ractl->_index + ractl->_nr_pages - index;
   continue;
  }
  if (i == mark)
   folio_set_readahead(folio);
  ractl->_workingset |= folio_test_workingset(folio);
  ractl->_nr_pages += min_nrpages;
  i += min_nrpages;
}

/*
* 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;

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
*/
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;
  do_page_cache_ra(ractl, this_chunk, 0);

  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.
*/

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 = ractl_alloc_folio(ractl, gfp, order);

if (!folio)
  return -ENOMEM;
mark = round_down(mark, 1UL << order);
if (index == mark)
  folio_set_readahead(folio);
err = filemap_add_folio(ractl->mapping, folio, index, gfp);
if (err) {
  folio_put(folio);
  return err;
}

ractl->_nr_pages += 1UL << order;
ractl->_workingset |= folio_test_workingset(folio);
return 0;
}

void page_cache_ra_order(struct readahead_control *ractl,
  struct file_ra_state *ra)
{
struct address_space *mapping = ractl->mapping;
pgoff_t start = readahead_index(ractl);
pgoff_t index = start;
unsigned int min_order = mapping_min_folio_order(mapping);
pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
pgoff_t mark = index + ra->size - ra->async_size;
unsigned int nofs;
int err = 0;
gfp_t gfp = readahead_gfp_mask(mapping);
unsigned int new_order = ra->order;

if (!mapping_large_folio_support(mapping)) {
  ra->order = 0;
  goto fallback;
}

limit = min(limit, index + ra->size - 1);

new_order = min(mapping_max_folio_order(mapping), new_order);
new_order = min_t(unsigned int, new_order, ilog2(ra->size));
new_order = max(new_order, min_order);

ra->order = new_order;

/* See comment in page_cache_ra_unbounded() */
nofs = memalloc_nofs_save();
filemap_invalidate_lock_shared(mapping);
/*
* If the new_order is greater than min_order and index is
* already aligned to new_order, then this will be noop as index
* aligned to new_order should also be aligned to min_order.
*/
ractl->_index = mapping_align_index(mapping, index);
index = readahead_index(ractl);

while (index <= limit) {
  unsigned int order = new_order;

  /* Align with smaller pages if needed */
  if (index & ((1UL << order) - 1))
   order = __ffs(index);
  /* Don't allocate pages past EOF */
  while (order > min_order && index + (1UL << order) - 1 > limit)
   order--;
  err = ra_alloc_folio(ractl, index, mark, order, gfp);
  if (err)
   break;
  index += 1UL << order;
}

read_pages(ractl);
filemap_invalidate_unlock_shared(mapping);
memalloc_nofs_restore(nofs);

/*
* 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 below.
*/
if (!err)
  return;
fallback:
/*
* ->readahead() may have updated readahead window size so we have to
* check there's still something to read.
*/
if (ra->size > index - start)
  do_page_cache_ra(ractl, ra->size - (index - start),
     ra->async_size);
}

static unsigned long ractl_max_pages(struct readahead_control *ractl,
  unsigned long req_size)
{
struct backing_dev_info *bdi = inode_to_bdi(ractl->mapping->host);
unsigned long max_pages = ractl->ra->ra_pages;

/*
* 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);
return max_pages;
}

void page_cache_sync_ra(struct readahead_control *ractl,
  unsigned long req_count)
{
pgoff_t index = readahead_index(ractl);
bool do_forced_ra = ractl->file && (ractl->file->f_mode & FMODE_RANDOM);
struct file_ra_state *ra = ractl->ra;
unsigned long max_pages, contig_count;
pgoff_t prev_index, miss;

/*
* 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 (!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;
}

max_pages = ractl_max_pages(ractl, req_count);
prev_index = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
/*
* A start of file, oversized read, or sequential cache miss:
* trivial case: (index - prev_index) == 1
* unaligned reads: (index - prev_index) == 0
*/
if (!index || req_count > max_pages || index - prev_index <= 1UL) {
  ra->start = index;
  ra->size = get_init_ra_size(req_count, max_pages);
  ra->async_size = ra->size > req_count ? ra->size - req_count :
       ra->size >> 1;
  goto readit;
}

/*
* Query the page cache and look for the traces(cached history pages)
* that a sequential stream would leave behind.
*/
rcu_read_lock();
miss = page_cache_prev_miss(ractl->mapping, index - 1, max_pages);
rcu_read_unlock();
contig_count = index - miss - 1;
/*
* Standalone, small random read. Read as is, and do not pollute the
* readahead state.
*/
if (contig_count <= req_count) {
  do_page_cache_ra(ractl, req_count, 0);
  return;
}
/*
* File cached from the beginning:
* it is a strong indication of long-run stream (or whole-file-read)
*/
if (miss == ULONG_MAX)
  contig_count *= 2;
ra->start = index;
ra->size = min(contig_count + req_count, max_pages);
ra->async_size = 1;
readit:
ra->order = 0;
ractl->_index = ra->start;
page_cache_ra_order(ractl, ra);
}
EXPORT_SYMBOL_GPL(page_cache_sync_ra);

void page_cache_async_ra(struct readahead_control *ractl,
  struct folio *folio, unsigned long req_count)
{
unsigned long max_pages;
struct file_ra_state *ra = ractl->ra;
pgoff_t index = readahead_index(ractl);
pgoff_t expected, start, end, aligned_end, align;

/* no readahead */
if (!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;

max_pages = ractl_max_pages(ractl, req_count);
/*
* It's the expected callback index, assume sequential access.
* Ramp up sizes, and push forward the readahead window.
*/
expected = round_down(ra->start + ra->size - ra->async_size,
   folio_nr_pages(folio));
if (index == expected) {
  ra->start += ra->size;
  /*
* In the case of MADV_HUGEPAGE, the actual size might exceed
* the readahead window.
*/
  ra->size = max(ra->size, get_next_ra_size(ra, max_pages));
  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.
*/
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_count;
ra->size = get_next_ra_size(ra, max_pages);
readit:
ra->order += 2;
align = 1UL << min(ra->order, ffs(max_pages) - 1);
end = ra->start + ra->size;
aligned_end = round_down(end, align);
if (aligned_end > ra->start)
  ra->size -= end - aligned_end;
ra->async_size = ra->size;
ractl->_index = ra->start;
page_cache_ra_order(ractl, ra);
}
EXPORT_SYMBOL_GPL(page_cache_async_ra);

ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
{
struct file *file;
const struct inode *inode;

CLASS(fd, f)(fd);
if (fd_empty(f))
  return -EBADF;

file = fd_file(f);
if (!(file->f_mode & FMODE_READ))
  return -EBADF;

/*
* 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.
*/
if (!file->f_mapping)
  return -EINVAL;
if (!file->f_mapping->a_ops)
  return -EINVAL;

inode = file_inode(file);
if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))
  return -EINVAL;
if (IS_ANON_FILE(inode))
  return -EINVAL;

return vfs_fadvise(fd_file(f), offset, count, POSIX_FADV_WILLNEED);
}

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);
unsigned long min_nrpages = mapping_min_folio_nrpages(mapping);
unsigned int min_order = mapping_min_folio_order(mapping);

new_index = new_start / PAGE_SIZE;
/*
* Readahead code should have aligned the ractl->_index to
* min_nrpages before calling readahead aops.
*/
VM_BUG_ON(!IS_ALIGNED(ractl->_index, min_nrpages));

/* Expand the leading edge downwards */
while (ractl->_index > new_index) {
  unsigned long index = ractl->_index - 1;
  struct folio *folio = xa_load(&mapping->i_pages, index);

  if (folio && !xa_is_value(folio))
   return; /* Folio apparently present */

  folio = ractl_alloc_folio(ractl, gfp_mask, min_order);
  if (!folio)
   return;

  index = mapping_align_index(mapping, index);
  if (filemap_add_folio(mapping, folio, index, gfp_mask) < 0) {
   folio_put(folio);
   return;
  }
  if (unlikely(folio_test_workingset(folio)) &&
    !ractl->_workingset) {
   ractl->_workingset = true;
   psi_memstall_enter(&ractl->_pflags);
  }
  ractl->_nr_pages += min_nrpages;
  ractl->_index = folio->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 folio *folio = xa_load(&mapping->i_pages, index);

  if (folio && !xa_is_value(folio))
   return; /* Folio apparently present */

  folio = ractl_alloc_folio(ractl, gfp_mask, min_order);
  if (!folio)
   return;

  index = mapping_align_index(mapping, index);
  if (filemap_add_folio(mapping, folio, index, gfp_mask) < 0) {
   folio_put(folio);
   return;
  }
  if (unlikely(folio_test_workingset(folio)) &&
    !ractl->_workingset) {
   ractl->_workingset = true;
   psi_memstall_enter(&ractl->_pflags);
  }
  ractl->_nr_pages += min_nrpages;
  if (ra) {
   ra->size += min_nrpages;
   ra->async_size += min_nrpages;
  }
}
}
EXPORT_SYMBOL(readahead_expand);

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