mm/readahead: fix large folio support in async readahead

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.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 4687fdb ("mm/filemap: Support VM_HUGEPAGE for file mappings")
Signed-off-by: Yafang Shao <[email protected]>
Tested-by: kernel test robot <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: laoar

mm/readahead.c

@@ -646,7 +646,11 @@ void page_cache_async_ra(struct readahead_control *ractl,
 			1UL << order);
 	if (index == expected) {
 		ra->start += ra->size;
-		ra->size = get_next_ra_size(ra, max_pages);
+		/*
+		 * 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));
 		ra->async_size = ra->size;
 		goto readit;
 	}