Suppose task @t that belongs to a memory cgroup @memcg is going to
allocate an object from a kmem cache @c. The copy of @c corresponding to
@memcg, @mc, is empty. Then if kmem_cache_alloc races with the memory
cgroup destruction we can access the memory cgroup's copy of the cache
after it was destroyed:
CPU0 CPU1
---- ----
[ current=@t
@mc->memcg_params->nr_pages=0 ]
kmem_cache_alloc(@c):
call memcg_kmem_get_cache(@c);
proceed to allocation from @mc:
alloc a page for @mc:
...
move @t from @memcg
destroy @memcg:
mem_cgroup_css_offline(@memcg):
memcg_unregister_all_caches(@memcg):
kmem_cache_destroy(@mc)
add page to @mc
We could fix this issue by taking a reference to a per-memcg cache, but
that would require adding a per-cpu reference counter to per-memcg caches,
which would look cumbersome.
Instead, let's take a reference to a memory cgroup, which already has a
per-cpu reference counter, in the beginning of kmem_cache_alloc to be
dropped in the end, and move per memcg caches destruction from css offline
to css free. As a side effect, per-memcg caches will be destroyed not one
by one, but all at once when the last page accounted to the memory cgroup
is freed. This doesn't sound as a high price for code readability though.
Note, this patch does add some overhead to the kmem_cache_alloc hot path,
but it is pretty negligible - it's just a function call plus a per cpu
counter decrement, which is comparable to what we already have in
memcg_kmem_get_cache. Besides, it's only relevant if there are memory
cgroups with kmem accounting enabled. I don't think we can find a way to
handle this race w/o it, because alloc_page called from kmem_cache_alloc
may sleep so we can't flush all pending kmallocs w/o reference counting.
Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
8135be5a80