[PATCH v2 1/3] mm: memcontrol: fix swap counter leak on swapout from offline cgroup

Vladimir Davydov
2016-08-02T17:42:23Z

An offline memory cgroup might have anonymous memory or shmem left
charged to it and no swap. Since only swap entries pin the id of an
offline cgroup, such a cgroup will have no id and so an attempt to
swapout its anon/shmem will not store memory cgroup info in the swap
cgroup map. As a result, memcg->swap or memcg->memsw will never get
uncharged from it and any of its ascendants.

Fix this by always charging swapout to the first ancestor cgroup that
hasn't released its id yet.

Fixes: 73f576c04b941 ("mm: memcontrol: fix cgroup creation failure after many small jobs")
Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Cc: <stable@vger.kernel.org> [3.19+]
---
Changes in v2:
- handle !use_hierarchy case properly (Michal)

mm/memcontrol.c | 38 ++++++++++++++++++++++++++++++++------
1 file changed, 32 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3be791afd372..4ae12effe347 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4036,6 +4036,24 @@ static void mem_cgroup_id_get(struct mem_cgroup *memcg)
atomic_inc(&memcg->id.ref);
}

+static struct mem_cgroup *mem_cgroup_id_get_active(struct mem_cgroup *memcg)
+{
+ while (!atomic_inc_not_zero(&memcg->id.ref)) {
+ /*
+ * The root cgroup cannot be destroyed, so it's refcount must
+ * always be >= 1.
+ */
+ if (memcg == root_mem_cgroup) {
+ VM_BUG_ON(1);
+ break;
+ }
+ memcg = parent_mem_cgroup(memcg);
+ if (!memcg)
+ memcg = root_mem_cgroup;
+ }
+ return memcg;
+}
+
static void mem_cgroup_id_put(struct mem_cgroup *memcg)
{
if (atomic_dec_and_test(&memcg->id.ref)) {
@@ -5752,7 +5770,7 @@ subsys_initcall(mem_cgroup_init);
*/
void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
{
- struct mem_cgroup *memcg;
+ struct mem_cgroup *memcg, *swap_memcg;
unsigned short oldid;

VM_BUG_ON_PAGE(PageLRU(page), page);
@@ -5767,15 +5785,20 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
if (!memcg)
return;

- mem_cgroup_id_get(memcg);
- oldid = swap_cgroup_record(entry, mem_cgroup_id(memcg));
+ swap_memcg = mem_cgroup_id_get_active(memcg);
+ oldid = swap_cgroup_record(entry, mem_cgroup_id(swap_memcg));
VM_BUG_ON_PAGE(oldid, page);
- mem_cgroup_swap_statistics(memcg, true);
+ mem_cgroup_swap_statistics(swap_memcg, true);

page->mem_cgroup = NULL;

if (!mem_cgroup_is_root(memcg))
page_counter_uncharge(&memcg->memory, 1);
+ if (memcg != swap_memcg) {
+ if (!mem_cgroup_is_root(swap_memcg))
+ page_counter_charge(&swap_memcg->memsw, 1);
+ page_counter_uncharge(&memcg->memsw, 1);
+ }

/*
* Interrupts should be disabled here because the caller holds the
@@ -5815,11 +5838,14 @@ int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
if (!memcg)
return 0;

+ memcg = mem_cgroup_id_get_active(memcg);
+
if (!mem_cgroup_is_root(memcg) &&
- !page_counter_try_charge(&memcg->swap, 1, &counter))
+ !page_counter_try_charge(&memcg->swap, 1, &counter)) {
+ mem_cgroup_id_put(memcg);
return -ENOMEM;
+ }

- mem_cgroup_id_get(memcg);
oldid = swap_cgroup_record(entry, mem_cgroup_id(memcg));
VM_BUG_ON_PAGE(oldid, page);
mem_cgroup_swap_statistics(memcg, true);
--
2.1.4