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@xxxxxxxxxxxxx>
CC: Michal Hocko <mhocko@xxxxxxx>
CC: Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
CC: Johannes Weiner <hannes@xxxxxxxxxxx>
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/memcontrol.h | 6 ++++++
mm/slab.c | 1 +
mm/slub.c | 21 +++++++++++++++++----
3 files changed, 24 insertions(+), 4 deletions(-)
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 17d0d41..48eddec 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -415,6 +415,12 @@ static inline void sock_release_memcg(struct sock *sk)
extern struct static_key memcg_kmem_enabled_key;
extern int memcg_limited_groups_array_size;
+
+/*
+ * Helper macro to loop through all memcg-specific caches. Callers must still
+ * check if the cache is valid (it is either valid or NULL).
+ * the slab_mutex must be held when looping through those caches
+ */
#define for_each_memcg_cache_index(_idx) \
for ((_idx) = 0; i < memcg_limited_groups_array_size; (_idx)++)
diff --git a/mm/slab.c b/mm/slab.c
index 699d1d42..d408bf731 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -4165,6 +4165,7 @@ 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 56a8db2..fead2cd 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5199,12 +5199,25 @@ 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);
}
--
1.7.11.7
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