[PATCH v2] mm/readahead: Fix large folio support in async readahead

[Yafang Shao <laoar.shao@gmail.com>]

When testing large folio support with XFS on our servers, we observed that
only a few large folios are mapped when reading large files via mmap.
After a thorough analysis, I identified it was caused by the
`/sys/block/*/queue/read_ahead_kb` setting. On our test servers, this
parameter is set to 128KB. After I tune it to 2MB, the large folio can
work as expected. However, I believe the large folio behavior should not be
dependent on the value of read_ahead_kb. It would be more robust if the
kernel can automatically adopt to it.

With /sys/block/*/queue/read_ahead_kb set to 128KB and performing a
sequential read on a 1GB file using MADV_HUGEPAGE, the differences in
/proc/meminfo are as follows:

- before this patch
  FileHugePages:     18432 kB
  FilePmdMapped:      4096 kB

- after this patch
  FileHugePages:   1067008 kB
  FilePmdMapped:   1048576 kB

This shows that after applying the patch, the entire 1GB file is mapped to
huge pages. The stable list is CCed, as without this patch, large folios
don’t function optimally in the readahead path. 

It's worth noting that if read_ahead_kb is set to a larger value that isn't
aligned with huge page sizes (e.g., 4MB + 128KB), it may still fail to map
to hugepages.

Fixes: 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file mappings")
Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: stable@vger.kernel.org

---
 mm/readahead.c | 2 ++
 1 file changed, 2 insertions(+)

Changes:
v1->v2:
- Drop the align (Matthew)
- Improve commit log (Andrew)

RFC->v1: https://lore.kernel.org/linux-mm/20241106092114.8408-1-laoar.shao@gmail.com/
- Simplify the code as suggested by Matthew

RFC: https://lore.kernel.org/linux-mm/20241104143015.34684-1-laoar.shao@gmail.com/

diff --git a/mm/readahead.c b/mm/readahead.c
index 3dc6c7a128dd..9b8a48e736c6 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -385,6 +385,8 @@ static unsigned long get_next_ra_size(struct file_ra_state *ra,
 		return 4 * cur;
 	if (cur <= max / 2)
 		return 2 * cur;
+	if (cur > max)
+		return cur;
 	return max;
 }
 
-- 
2.43.5