On Mon 01-08-16 16:26:24, 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.
Ahh, OK, very good point! Multiple ref. counts are always very
confusing. I guess it would be good to mention the impact which would
be pre-mature memcg oom or excessive reclaim in deeper hierarchies with
the memsw limit or the swap limit enforced in a upper hierarchy AFAIU.
v2 swap limit would be little bit more subtle because it would prevent
swapout too early.
> Fix this by always charging swapout to the first ancestor cgroup that
> hasn't released its id yet.
This is a bit ugly but I guess the easiest way to fix this.
> Fixes: 73f576c04b941 ("mm: memcontrol: fix cgroup creation failure after many small jobs")
The original commit was marked for stable so this should go to stable as
well. I already have the above backported for 4.4 so weill queue this
one up as well when submitting.
> Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!
> ---
> mm/memcontrol.c | 27 +++++++++++++++++++++------
> 1 file changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b5804e4e6324..5fe285f27ea7 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4035,6 +4035,13 @@ 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))
> + memcg = parent_mem_cgroup(memcg);
> + return memcg;
> +}
> +
> static void mem_cgroup_id_put(struct mem_cgroup *memcg)
> {
> if (atomic_dec_and_test(&memcg->id.ref)) {
> @@ -5751,7 +5758,7 @@ 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);
> @@ -5766,15 +5773,20 @@ 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
> @@ -5814,11 +5826,14 @@ 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
>
> --
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--
Michal Hocko
SUSE Labs
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