Patch "dm bufio: fix shrinker scans when (nr_to_scan < retain_target)" has been added to the 4.9-stable tree

Patch "dm bufio: fix shrinker scans when (nr_to_scan < retain_target)" has been added to the 4.9-stable tree

[gregkh]

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 4.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

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-4.9 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 |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -1554,7 +1554,8 @@ 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++) {
@@ -1591,6 +1592,7 @@ dm_bufio_shrink_count(struct shrinker *s
 {
 	struct dm_bufio_client *c;
 	unsigned long count;
+	unsigned long retain_target;
 
 	c = container_of(shrink, struct dm_bufio_client, shrinker);
 	if (sc->gfp_mask & __GFP_FS)
@@ -1599,8 +1601,9 @@ dm_bufio_shrink_count(struct shrinker *s
 		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;
+	return (count < retain_target) ? 0 : (count - retain_target);
 }
 
 /*


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

queue-4.9/dm-bufio-fix-shrinker-scans-when-nr_to_scan-retain_target.patch