[PATCH] mm: readahead: improve mmap_miss heuristic for concurrent faults

[Roman Gushchin <roman.gushchin@linux.dev>]

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.

Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: linux-mm@kvack.org
---
 mm/filemap.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index c21e98657e0b..983ba1019674 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3324,9 +3324,17 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
 	if (vmf->vma->vm_flags & VM_RAND_READ || !ra->ra_pages)
 		return fpin;
 
-	mmap_miss = READ_ONCE(ra->mmap_miss);
-	if (mmap_miss)
-		WRITE_ONCE(ra->mmap_miss, --mmap_miss);
+	/*
+	 * 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);
-- 
2.50.1