From ebe945c27628fca03723582eba138acc2e2f3d15 Mon Sep 17 00:00:00 2001 From: Glauber Costa Date: Tue, 18 Dec 2012 14:23:10 -0800 Subject: 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 Cc: Michal Hocko Cc: Kamezawa Hiroyuki Cc: Johannes Weiner Acked-by: David Rientjes Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/slub.c | 21 +++++++++++++++++---- 1 file changed, 17 insertions(+), 4 deletions(-) (limited to 'mm/slub.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); } -- cgit v1.2.2