[PATCH -mm] slub: kmem_cache_shrink: check if partial list is empty under list_lock

[Vladimir Davydov]

SLUB's implementation of kmem_cache_shrink skips nodes that have
nr_partial=0, because they surely don't have any empty slabs to free.
This check is done w/o holding any locks, therefore it can race with
concurrent kfree adding an empty slab to a partial list. As a result, a
just shrinked cache can have empty slabs.

This is unacceptable for kmemcg, which needs to be sure that there will
be no empty slabs on dead memcg caches after kmem_cache_shrink was
called, because otherwise we may leak a dead cache.

Let's fix this race by checking if node partial list is empty under
node->list_lock. Since the nr_partial!=0 branch of kmem_cache_shrink
does nothing if the list is empty, we can simply remove the nr_partial=0
check.

Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
Reported-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
---
 mm/slub.c |    3 ---
 1 file changed, 3 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 67da14d9ec70..891ac6cd78cc 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3397,9 +3397,6 @@ int __kmem_cache_shrink(struct kmem_cache *s)
 
 	flush_all(s);
 	for_each_kmem_cache_node(s, node, n) {
-		if (!n->nr_partial)
-			continue;
-
 		for (i = 0; i < objects; i++)
 			INIT_LIST_HEAD(slabs_by_inuse + i);
 
-- 
1.7.10.4

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