This is a note to let you know that I've just added the patch titled

dm bufio: fix shrinker scans when (nr_to_scan < retain_target)

to the 4.14-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: dm-bufio-fix-shrinker-scans-when-nr_to_scan-retain_target.patch and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From fbc7c07ec23c040179384a1f16b62b6030eb6bdd Mon Sep 17 00:00:00 2001

From: Suren Baghdasaryan surenb@google.com Date: Wed, 6 Dec 2017 09:27:30 -0800 Subject: dm bufio: fix shrinker scans when (nr_to_scan < retain_target)

From: Suren Baghdasaryan surenb@google.com

commit fbc7c07ec23c040179384a1f16b62b6030eb6bdd upstream.

When system is under memory pressure it is observed that dm bufio shrinker often reclaims only one buffer per scan. This change fixes the following two issues in dm bufio shrinker that cause this behavior:

1. ((nr_to_scan - freed) <= retain_target) condition is used to terminate slab scan process. This assumes that nr_to_scan is equal to the LRU size, which might not be correct because do_shrink_slab() in vmscan.c calculates nr_to_scan using multiple inputs. As a result when nr_to_scan is less than retain_target (64) the scan will terminate after the first iteration, effectively reclaiming one buffer per scan and making scans very inefficient. This hurts vmscan performance especially because mutex is acquired/released every time dm_bufio_shrink_scan() is called. New implementation uses ((LRU size - freed) <= retain_target) condition for scan termination. LRU size can be safely determined inside __scan() because this function is called after dm_bufio_lock().

2. do_shrink_slab() uses value returned by dm_bufio_shrink_count() to determine number of freeable objects in the slab. However dm_bufio always retains retain_target buffers in its LRU and will terminate a scan when this mark is reached. Therefore returning the entire LRU size from dm_bufio_shrink_count() is misleading because that does not represent the number of freeable objects that slab will reclaim during a scan. Returning (LRU size - retain_target) better represents the number of freeable objects in the slab. This way do_shrink_slab() returns 0 when (LRU size < retain_target) and vmscan will not try to scan this shrinker avoiding scans that will not reclaim any memory.

Test: tested using Android device running <AOSP>/system/extras/alloc-stress that generates memory pressure and causes intensive shrinker scans

Signed-off-by: Suren Baghdasaryan surenb@google.com Signed-off-by: Mike Snitzer snitzer@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

--- drivers/md/dm-bufio.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/drivers/md/dm-bufio.c +++ b/drivers/md/dm-bufio.c @@ -1611,7 +1611,8 @@ static unsigned long __scan(struct dm_bu int l; struct dm_buffer *b, *tmp; unsigned long freed = 0; - unsigned long count = nr_to_scan; + unsigned long count = c->n_buffers[LIST_CLEAN] + + c->n_buffers[LIST_DIRTY]; unsigned long retain_target = get_retain_buffers(c);

for (l = 0; l < LIST_SIZE; l++) { @@ -1647,8 +1648,11 @@ static unsigned long dm_bufio_shrink_count(struct shrinker *shrink, struct shrink_control *sc) { struct dm_bufio_client *c = container_of(shrink, struct dm_bufio_client, shrinker); + unsigned long count = ACCESS_ONCE(c->n_buffers[LIST_CLEAN]) + + ACCESS_ONCE(c->n_buffers[LIST_DIRTY]); + unsigned long retain_target = get_retain_buffers(c);

- return ACCESS_ONCE(c->n_buffers[LIST_CLEAN]) + ACCESS_ONCE(c->n_buffers[LIST_DIRTY]); + return (count < retain_target) ? 0 : (count - retain_target); }

/*

Patches currently in stable-queue which might be from surenb@google.com are

queue-4.14/dm-bufio-fix-shrinker-scans-when-nr_to_scan-retain_target.patch