This is backport of the upstream patch d12067f428c037b4575aaeb2be00847fc214c24a. It should be backported to stable kernels because this performance problem was seen on the android 4.4 kernel.

commit d12067f428c037b4575aaeb2be00847fc214c24a Author: Mikulas Patocka mpatocka@redhat.com Date: Wed Nov 23 16:52:01 2016 -0500

dm bufio: don't take the lock in dm_bufio_shrink_count

dm_bufio_shrink_count() is called from do_shrink_slab to find out how many freeable objects are there. The reported value doesn't have to be precise, so we don't need to take the dm-bufio lock.

Suggested-by: David Rientjes rientjes@google.com Signed-off-by: Mikulas Patocka mpatocka@redhat.com Signed-off-by: Mike Snitzer snitzer@redhat.com

--- drivers/md/dm-bufio.c | 16 ++++------------ 1 file changed, 4 insertions(+), 12 deletions(-)

Index: linux-stable/drivers/md/dm-bufio.c =================================================================== --- linux-stable.orig/drivers/md/dm-bufio.c 2018-07-04 15:25:41.000000000 +0200 +++ linux-stable/drivers/md/dm-bufio.c 2018-07-04 15:33:59.000000000 +0200 @@ -1574,19 +1574,11 @@ dm_bufio_shrink_scan(struct shrinker *sh static unsigned long dm_bufio_shrink_count(struct shrinker *shrink, struct shrink_control *sc) { - struct dm_bufio_client *c; - unsigned long count; - unsigned long retain_target; + struct dm_bufio_client *c = container_of(shrink, struct dm_bufio_client, shrinker); + unsigned long count = READ_ONCE(c->n_buffers[LIST_CLEAN]) + + READ_ONCE(c->n_buffers[LIST_DIRTY]); + unsigned long retain_target = get_retain_buffers(c);

- c = container_of(shrink, struct dm_bufio_client, shrinker); - if (sc->gfp_mask & __GFP_FS) - dm_bufio_lock(c); - else if (!dm_bufio_trylock(c)) - return 0; - - count = c->n_buffers[LIST_CLEAN] + c->n_buffers[LIST_DIRTY]; - retain_target = get_retain_buffers(c); - dm_bufio_unlock(c); return (count < retain_target) ? 0 : (count - retain_target); }