mm: memcg/slab: fix obj_cgroup_charge() return value handling

Commit 10befea91b ("mm: memcg/slab: use a single set of kmem_caches
for all allocations") introduced a regression into the handling of the
obj_cgroup_charge() return value.  If a non-zero value is returned
(indicating of exceeding one of memory.max limits), the allocation
should fail, instead of falling back to non-accounted mode.

To make the code more readable, move memcg_slab_pre_alloc_hook() and
memcg_slab_post_alloc_hook() calling conditions into bodies of these
hooks.

Fixes: 10befea91b ("mm: memcg/slab: use a single set of kmem_caches for all allocations")
Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20201127161828.GD840171@carbon.dhcp.thefacebook.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slab.h

@@ -274,22 +274,32 @@ static inline size_t obj_full_size(struct kmem_cache *s)
 	return s->size + sizeof(struct obj_cgroup *);
 }
 
-static inline struct obj_cgroup *memcg_slab_pre_alloc_hook(struct kmem_cache *s,
-							   size_t objects,
-							   gfp_t flags)
+/*
+ * Returns false if the allocation should fail.
+ */
+static inline bool memcg_slab_pre_alloc_hook(struct kmem_cache *s,
+					     struct obj_cgroup **objcgp,
+					     size_t objects, gfp_t flags)
 {
 	struct obj_cgroup *objcg;
 
+	if (!memcg_kmem_enabled())
+		return true;
+
+	if (!(flags & __GFP_ACCOUNT) && !(s->flags & SLAB_ACCOUNT))
+		return true;
+
 	objcg = get_obj_cgroup_from_current();
 	if (!objcg)
-		return NULL;
+		return true;
 
 	if (obj_cgroup_charge(objcg, flags, objects * obj_full_size(s))) {
 		obj_cgroup_put(objcg);
-		return NULL;
+		return false;
 	}
 
-	return objcg;
+	*objcgp = objcg;
+	return true;
 }
 
 static inline void mod_objcg_state(struct obj_cgroup *objcg,
@@ -315,7 +325,7 @@ static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s,
 	unsigned long off;
 	size_t i;
 
-	if (!objcg)
+	if (!memcg_kmem_enabled() || !objcg)
 		return;
 
 	flags &= ~__GFP_ACCOUNT;
@@ -400,11 +410,11 @@ static inline void memcg_free_page_obj_cgroups(struct page *page)
 {
 }
 
-static inline struct obj_cgroup *memcg_slab_pre_alloc_hook(struct kmem_cache *s,
-							   size_t objects,
-							   gfp_t flags)
+static inline bool memcg_slab_pre_alloc_hook(struct kmem_cache *s,
+					     struct obj_cgroup **objcgp,
+					     size_t objects, gfp_t flags)
 {
-	return NULL;
+	return true;
 }
 
 static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s,
@@ -508,9 +518,8 @@ static inline struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s,
 	if (should_failslab(s, flags))
 		return NULL;
 
-	if (memcg_kmem_enabled() &&
-	    ((flags & __GFP_ACCOUNT) || (s->flags & SLAB_ACCOUNT)))
-		*objcgp = memcg_slab_pre_alloc_hook(s, size, flags);
+	if (!memcg_slab_pre_alloc_hook(s, objcgp, size, flags))
+		return NULL;
 
 	return s;
 }
@@ -529,8 +538,7 @@ static inline void slab_post_alloc_hook(struct kmem_cache *s,
 					 s->flags, flags);
 	}
 
-	if (memcg_kmem_enabled())
-		memcg_slab_post_alloc_hook(s, objcg, flags, size, p);
+	memcg_slab_post_alloc_hook(s, objcg, flags, size, p);
 }
 
 #ifndef CONFIG_SLOB