memcg: add comments clarifying aspects of cache attribute propagation

This patch clarifies two aspects of cache attribute propagation.

First, the expected context for the for_each_memcg_cache macro in
memcontrol.h.  The usages already in the codebase are safe.  In mm/slub.c,
it is trivially safe because the lock is acquired right before the loop.
In mm/slab.c, it is less so: the lock is acquired by an outer function a
few steps back in the stack, so a VM_BUG_ON() is added to make sure it is
indeed safe.

A comment is also added to detail why we are returning the value of the
parent cache and ignoring the children's when we propagate the attributes.

Signed-off-by: Glauber Costa <glommer@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 18 insertions, 4 deletions

diff --git a/mm/slab.c b/mm/slab.c
index 4dcbf96a77b4..e7667a3584bc 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -4099,6 +4099,7 @@ static int do_tune_cpucache(struct kmem_cache *cachep, int limit,
         if ((ret < 0) || !is_root_cache(cachep))
                 return ret;
 
+        VM_BUG_ON(!mutex_is_locked(&slab_mutex));
         for_each_memcg_cache_index(i) {
                 c = cache_from_memcg(cachep, i);
                 if (c)
diff --git a/mm/slub.c b/mm/slub.c
index 21c94d9695ec..efe2cffc29b0 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5108,12 +5108,25 @@ static ssize_t slab_attr_store(struct kobject *kobj,
                 if (s->max_attr_size < len)
                         s->max_attr_size = len;
 
+                /*
+                 * This is a best effort propagation, so this function's return
+                 * value will be determined by the parent cache only. This is
+                 * basically because not all attributes will have a well
+                 * defined semantics for rollbacks - most of the actions will
+                 * have permanent effects.
+                 *
+                 * Returning the error value of any of the children that fail
+                 * is not 100 % defined, in the sense that users seeing the
+                 * error code won't be able to know anything about the state of
+                 * the cache.
+                 *
+                 * Only returning the error code for the parent cache at least
+                 * has well defined semantics. The cache being written to
+                 * directly either failed or succeeded, in which case we loop
+                 * through the descendants with best-effort propagation.
+                 */
                 for_each_memcg_cache_index(i) {
                         struct kmem_cache *c = cache_from_memcg(s, i);
-                        /*
-                         * This function's return value is determined by the
-                         * parent cache only
-                         */
                         if (c)
                                 attribute->store(c, buf, len);
                 }