Commit e338d835 authored by Roman Gushchin's avatar Roman Gushchin Committed by Andrew Morton
Browse files

mm: readahead: improve mmap_miss heuristic for concurrent faults 

If two or more threads of an application faulting on the same folio, the
mmap_miss counter can be decreased multiple times.  It breaks the
mmap_miss heuristic and keeps the readahead enabled even under extreme
levels of memory pressure.

It happens often if file folios backing a multi-threaded application are
getting evicted and re-faulted.

Fix it by skipping decreasing mmap_miss if the folio is locked.

This change was evaluated on several hundred thousands hosts in Google's
production over a couple of weeks.  The number of containers being stuck
in a vicious reclaim cycle for a long time was reduced several fold
(~10-20x), as well as the overall fleet-wide cpu time spent in direct
memory reclaim was meaningfully reduced.  No regressions were observed.

Link: https://lkml.kernel.org/r/20250815183224.62007-1-roman.gushchin@linux.dev


Signed-off-by: default avatarRoman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: default avatarJan Kara <jack@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 19de1e5d

mm/filemap.c

+11 −3
Original line number Diff line number Diff line
@@ -3323,9 +3323,17 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf, 
	if (vmf->vma->vm_flags & VM_RAND_READ || !ra->ra_pages)

		return fpin;



	/*

	 * If the folio is locked, we're likely racing against another fault.

	 * Don't touch the mmap_miss counter to avoid decreasing it multiple

	 * times for a single folio and break the balance with mmap_miss

	 * increase in do_sync_mmap_readahead().

	 */

	if (likely(!folio_test_locked(folio))) {

		mmap_miss = READ_ONCE(ra->mmap_miss);

		if (mmap_miss)

			WRITE_ONCE(ra->mmap_miss, --mmap_miss);

	}



	if (folio_test_readahead(folio)) {

		fpin = maybe_unlock_mmap_for_io(vmf, fpin);