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SubjectRe: [patch] mm: memcg: update the correct soft limit tree during migration
On Fri 13-01-12 16:41:31, Johannes Weiner wrote:
> end_migration() passes the old page instead of the new page to commit
> the charge. This page descriptor is not used for committing itself,
> though, since we also pass the (correct) page_cgroup descriptor. But
> it's used to find the soft limit tree through the page's zone, so the
> soft limit tree of the old page's zone is updated instead of that of
> the new page's, which might get slightly out of date until the next
> charge reaches the ratelimit point.
>
> This glitch has been present since '5564e88 memcg: condense
> page_cgroup-to-page lookup points'.
>
> Reported-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Well spotted.
Acked-by: Michal Hocko <mhocko@suse.cz>

Thanks
> ---
> mm/memcontrol.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> This fixes a bug that I introduced in 2.6.38. It's benign enough (to
> my knowledge) that we probably don't want this for stable.
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 602207b..7a292a5 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3247,7 +3247,7 @@ int mem_cgroup_prepare_migration(struct page *page,
> ctype = MEM_CGROUP_CHARGE_TYPE_CACHE;
> else
> ctype = MEM_CGROUP_CHARGE_TYPE_SHMEM;
> - __mem_cgroup_commit_charge(memcg, page, 1, pc, ctype);
> + __mem_cgroup_commit_charge(memcg, newpage, 1, pc, ctype);
> return ret;
> }
>
> --
> 1.7.7.5
>

--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic


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