[PATCH 4.4, 4.9] dm bufio: don't take the lock in dm_bufio_shrink_count

[PATCH 4.4, 4.9] dm bufio: don't take the lock in dm_bufio_shrink_count

[Mikulas Patocka]

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);
 }
 

Re: [PATCH 4.4, 4.9] dm bufio: don't take the lock in dm_bufio_shrink_count

[Greg Kroah-Hartman]

On Wed, Jul 04, 2018 at 10:41:03AM -0400, Mikulas Patocka wrote:
> 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.

Now applied, thanks.

greg k-h