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Commit fcff7d7e authored by Vladimir Davydov's avatar Vladimir Davydov Committed by Linus Torvalds
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mm: memcontrol: do not bypass slab charge if memcg is offline 



Slab pages are charged in two steps.  First, an appropriate per memcg
cache is selected (see memcg_kmem_get_cache) basing on the current
context, then the new slab page is charged to the memory cgroup which
the selected cache was created for (see memcg_charge_slab ->
__memcg_kmem_charge_memcg).  It is OK to bypass kmemcg charge at step 1,
but if step 1 succeeded and we successfully allocated a new slab page,
step 2 must be performed, otherwise we would get a per memcg kmem cache
which contains a slab that does not hold a reference to the memory
cgroup owning the cache.  Since per memcg kmem caches are destroyed on
memcg css free, this could result in freeing a cache while there are
still active objects in it.

However, currently we will bypass slab page charge if the memory cgroup
owning the cache is offline (see __memcg_kmem_charge_memcg).  This is
very unlikely to occur in practice, because for this to happen a process
must be migrated to a different cgroup and the old cgroup must be
removed while the process is in kmalloc somewhere between steps 1 and 2
(e.g.  trying to allocate a new page).  Nevertheless, it's still better
to eliminate such a possibility.

Signed-off-by: default avatarVladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 6a618957

mm/memcontrol.c

+3 −5
Original line number Diff line number Diff line
@@ -2325,9 +2325,6 @@ int __memcg_kmem_charge_memcg(struct page *page, gfp_t gfp, int order, 
	struct page_counter *counter;

	int ret;



	if (!memcg_kmem_online(memcg))

		return 0;



	ret = try_charge(memcg, gfp, nr_pages);

	if (ret)

		return ret;
@@ -2346,9 +2343,10 @@ int __memcg_kmem_charge_memcg(struct page *page, gfp_t gfp, int order, 
int __memcg_kmem_charge(struct page *page, gfp_t gfp, int order)

{

	struct mem_cgroup *memcg;

	int ret;

	int ret = 0;



	memcg = get_mem_cgroup_from_mm(current->mm);

	if (memcg_kmem_online(memcg))

		ret = __memcg_kmem_charge_memcg(page, gfp, order, memcg);

	css_put(&memcg->css);

	return ret;