[PATCH RFC 1/6] mm, slab: make caches with refcount of 0 unmergeable

[Vlastimil Babka <vbabka@suse.cz>]

Slab caches with refcount 0 are in the process of being destroyed so
it's undesirable for new caches to attempt merging with them. A
synchronous destruction happens under slab_mutex thus excluding
concurrent cache creation and merging. Full destruction of
SLAB_TYPESAFE_BY_RCU caches might be delayed, but the cache is still
taken off the slab_caches list immediately, thus unreachable by cache
creation.

However a cache where __kmem_cache_shutdown() fails because it contains
objects that were not freed (due to a bug in the cache user) will be
left on the slab_caches list and might be considered for merging.
Also the following patches will introduce a possibility of a cache with
refcount 0 being temporarily reachable on the slab_list even in case of
no bugs, due to kfree_rcu() in flight.

For these reasons, prevent merging with caches that have zero refcount.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slab_common.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 70943a4c1c4b..3ba205bda95d 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -150,9 +150,11 @@ int slab_unmergeable(struct kmem_cache *s)
 #endif
 
 	/*
-	 * We may have set a slab to be unmergeable during bootstrap.
+	 * We may have set a cache to be unmergeable (-1) during bootstrap.
+	 * 0 is for cache being destroyed asynchronously, or cache that failed
+	 * to destroy due to outstanding objects.
 	 */
-	if (s->refcount < 0)
+	if (s->refcount <= 0)
 		return 1;
 
 	return 0;

-- 
2.45.2