[PATCH] mm: consider disabling readahead if there are signs of thrashing

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

We've noticed in production that under a very heavy memory pressure
the readahead behavior becomes unstable causing spikes in memory
pressure and CPU contention on zone locks.

The current mmap_miss heuristics considers minor pagefaults as a
good reason to decrease mmap_miss and conditionally start async
readahead. This creates a vicious cycle: asynchronous readahead
loads more pages, which in turn causes more minor pagefaults.
This problem is especially pronounced when multiple threads of
an application fault on consecutive pages of an evicted executable,
aggressively lowering the mmap_miss counter and preventing readahead
from being disabled.

To improve the logic let's check for !uptodate and workingset
folios in do_async_mmap_readahead(). The presence of such pages
is a strong indicator of thrashing, which is also used by the
delay accounting code, e.g. in folio_wait_bit_common(). So instead
of decreasing mmap_miss and lower chances to disable readahead,
let's do the opposite and bump it by MMAP_LOTSAMISS / 2.

Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Liu Shixin <liushixin2@huawei.com>
Cc: linux-mm@kvack.org
---
 mm/filemap.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index 0d0369fb5fa1..ec3f611c3320 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3324,6 +3324,17 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
 		return fpin;
 
 	mmap_miss = READ_ONCE(ra->mmap_miss);
+	if (unlikely(!folio_test_uptodate(folio) &&
+		     folio_test_workingset(folio))) {
+		/*
+		 * If there are signs of thrashing, take a big step
+		 * towards disabling readahead.
+		 */
+		mmap_miss += MMAP_LOTSAMISS / 2;
+		mmap_miss = min(mmap_miss, MMAP_LOTSAMISS * 10);
+		WRITE_ONCE(ra->mmap_miss, mmap_miss);
+		return fpin;
+	}
 	if (mmap_miss)
 		WRITE_ONCE(ra->mmap_miss, --mmap_miss);
 
-- 
2.50.0