[PATCH 1/4] mm/readahead: fix the broken readahead for POSIX_FADV_WILLNEED

Jaegeuk Kim
2025-12-01T19:19:47Z

This patch fixes the broken readahead flow for POSIX_FADV_WILLNEED, where
the problem is, in force_page_cache_ra(nr_to_read), nr_to_read is cut by
the below code.

max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
nr_to_read = min_t(unsigned long, nr_to_read, max_pages);

IOWs, we are not able to read ahead larger than the above max_pages which
is most likely the range of 2MB and 16MB. Note, it doesn't make sense
to set ra->ra_pages to the entire file size. Instead, let's fix this logic.

Before:
f2fs_fadvise: dev = (252,16), ino = 14, i_size = 4294967296 offset:0, len:4294967296, advise:3
page_cache_ra_unbounded: dev=252:16 ino=e index=0 nr_to_read=512 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=512 nr_to_read=512 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=1024 nr_to_read=512 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=1536 nr_to_read=512 lookahead_size=0

After:
f2fs_fadvise: dev = (252,16), ino = 14, i_size = 4294967296 offset:0, len:4294967296, advise:3
page_cache_ra_unbounded: dev=252:16 ino=e index=0 nr_to_read=2048 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=2048 nr_to_read=2048 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=4096 nr_to_read=2048 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=6144 nr_to_read=2048 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=8192 nr_to_read=2048 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=10240 nr_to_read=2048 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=12288 nr_to_read=2048 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=14336 nr_to_read=2048 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=16384 nr_to_read=2048 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=18432 nr_to_read=2048 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=20480 nr_to_read=2048 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=22528 nr_to_read=2048 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=24576 nr_to_read=2048 lookahead_size=0
...
page_cache_ra_unbounded: dev=252:16 ino=e index=1042432 nr_to_read=2048 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=1044480 nr_to_read=2048 lookahead_size=0
page_cache_ra_unbounded: dev=252:16 ino=e index=1046528 nr_to_read=2048 lookahead_size=0

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
---
mm/readahead.c | 27 ++++++++++++---------------
1 file changed, 12 insertions(+), 15 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 3a4b5d58eeb6..c0db049a5b7b 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -311,7 +311,7 @@ EXPORT_SYMBOL_GPL(page_cache_ra_unbounded);
* 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,
+static int do_page_cache_ra(struct readahead_control *ractl,
unsigned long nr_to_read, unsigned long lookahead_size)
{
struct inode *inode = ractl->mapping->host;
@@ -320,45 +320,42 @@ static void do_page_cache_ra(struct readahead_control *ractl,
pgoff_t end_index; /* The last page we want to read */

if (isize == 0)
- return;
+ return -EINVAL;

end_index = (isize - 1) >> PAGE_SHIFT;
if (index > end_index)
- return;
+ return -EINVAL;
/* 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);
+ return 0;
}

/*
- * Chunk the readahead into 2 megabyte units, so that we don't pin too much
- * memory at once.
+ * Chunk the readahead per the block device capacity, and read all nr_to_read.
*/
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;
+ unsigned long this_chunk;

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
+ * Consier the optimal hardware IO size for readahead chunk.
*/
- max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
- nr_to_read = min_t(unsigned long, nr_to_read, max_pages);
+ this_chunk = max_t(unsigned long, bdi->io_pages, ractl->ra->ra_pages);
+
while (nr_to_read) {
- unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
+ this_chunk = min_t(unsigned long, this_chunk, nr_to_read);

- if (this_chunk > nr_to_read)
- this_chunk = nr_to_read;
- do_page_cache_ra(ractl, this_chunk, 0);
+ if (do_page_cache_ra(ractl, this_chunk, 0))
+ break;

nr_to_read -= this_chunk;