[PATCH v2] memcg: Prevent memcg caches to be both OFF_SLAB & OBJFREELIST_SLAB

[Thomas Garnier]

While testing OBJFREELIST_SLAB integration with pagealloc, we found a
bug where kmem_cache(sys) would be created with both CFLGS_OFF_SLAB &
CFLGS_OBJFREELIST_SLAB.

The original kmem_cache is created early making OFF_SLAB not possible.
When kmem_cache(sys) is created, OFF_SLAB is possible and if pagealloc
is enabled it will try to enable it first under certain conditions.
Given kmem_cache(sys) reuses the original flag, you can have both flags
at the same time resulting in allocation failures and odd behaviors.

This fix discards allocator specific flags from memcg and ensure
cache_create cannot be called with them.

Fixes: b03a017bebc4 ("mm/slab: introduce new slab management type, OBJFREELIST_SLAB")
Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx>
---
Based on next-20161025
---
 mm/slab.h        |  3 +++
 mm/slab_common.c | 10 ++++++++--
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/mm/slab.h b/mm/slab.h
index 9653f2e..58be647 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -144,6 +144,9 @@ static inline unsigned long kmem_cache_flags(unsigned long object_size,
 
 #define CACHE_CREATE_MASK (SLAB_CORE_FLAGS | SLAB_DEBUG_FLAGS | SLAB_CACHE_FLAGS)
 
+/* Common allocator flags allowed for cache_create. */
+#define SLAB_FLAGS_PERMITTED (CACHE_CREATE_MASK | SLAB_KASAN)
+
 int __kmem_cache_shutdown(struct kmem_cache *);
 void __kmem_cache_release(struct kmem_cache *);
 int __kmem_cache_shrink(struct kmem_cache *, bool);
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 71f0b28..01d067c 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -329,6 +329,12 @@ static struct kmem_cache *create_cache(const char *name,
 	struct kmem_cache *s;
 	int err;
 
+	/* Do not allow allocator specific flags */
+	if (flags & ~SLAB_FLAGS_PERMITTED) {
+		err = -EINVAL;
+		goto out;
+	}
+
 	err = -ENOMEM;
 	s = kmem_cache_zalloc(kmem_cache, GFP_KERNEL);
 	if (!s)
@@ -533,8 +539,8 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
 
 	s = create_cache(cache_name, root_cache->object_size,
 			 root_cache->size, root_cache->align,
-			 root_cache->flags, root_cache->ctor,
-			 memcg, root_cache);
+			 root_cache->flags & SLAB_FLAGS_PERMITTED,
+			 root_cache->ctor, memcg, root_cache);
 	/*
 	 * If we could not create a memcg cache, do not complain, because
 	 * that's not critical at all as we can always proceed with the root
-- 
2.8.0.rc3.226.g39d4020