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@xxxxxxxxxxxxx>
---
mm/memcontrol.c | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3e4199830456..f36b20f5b3ed 100644
--- 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;
}
--
2.1.4
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