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: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 42882c1e7fcea1a5896a77d58af2e6e87aabc898..5c9d45e4c739d8251201c6f005c13b59926155e0 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -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;
@@ -2346,10 +2343,11 @@ 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);
-       ret = __memcg_kmem_charge_memcg(page, gfp, order, memcg);
+       if (memcg_kmem_online(memcg))
+               ret = __memcg_kmem_charge_memcg(page, gfp, order, memcg);
        css_put(&memcg->css);
        return ret;
 }