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

Michal Hocko
2016-08-02T16:14:33Z

On Tue 02-08-16 18:00:48, Vladimir Davydov wrote:
> 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;
> + }

why not simply VM_BUG_ON(memcg == root_mem_cgroup)?

> + 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

The resulting code is a weird mixture of memcg and swap_memcg usage
which is really confusing and error prone. Do we really have to do
uncharge on an already offline memcg?
--
Michal Hocko
SUSE Labs