I am adding Vladimir to CC
On Wed 30-04-14 16:25:41, Johannes Weiner wrote:
> Kmem page charging and uncharging is serialized by means of exclusive
> access to the page. Do not take the page_cgroup lock and don't set
> pc->flags atomically.
>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
The patch is correct I just have some comments below.
Anyway
Acked-by: Michal Hocko <mhocko@xxxxxxx>
> ---
> mm/memcontrol.c | 16 +++-------------
> 1 file changed, 3 insertions(+), 13 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c528ae9ac230..d3961fce1d54 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3535,10 +3535,8 @@ void __memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg,
> }
>
/*
* given page is newly allocated and invisible to everybody but
* the caller so there is no need to use page_cgroup lock nor
* SetPageCgroupUsed
*/
would be helpful?
> pc = lookup_page_cgroup(page);
> - lock_page_cgroup(pc);
> pc->mem_cgroup = memcg;
> - SetPageCgroupUsed(pc);
> - unlock_page_cgroup(pc);
> + pc->flags = PCG_USED;
> }
>
> void __memcg_kmem_uncharge_pages(struct page *page, int order)
> @@ -3548,19 +3546,11 @@ void __memcg_kmem_uncharge_pages(struct page *page, int order)
>
>
> pc = lookup_page_cgroup(page);
> - /*
> - * Fast unlocked return. Theoretically might have changed, have to
> - * check again after locking.
> - */
This comment was there since the code has been merged. Maybe it was true
at the time but after "mm: get rid of __GFP_KMEMCG" it is definitely out
of date.
/*
* the pages is going away and will be freed and nobody can see
* it anymore so no need to take page_cgroup lock.
*/
> if (!PageCgroupUsed(pc))
> return;
>
> - lock_page_cgroup(pc);
> - if (PageCgroupUsed(pc)) {
> - memcg = pc->mem_cgroup;
> - ClearPageCgroupUsed(pc);
> - }
> - unlock_page_cgroup(pc);
maybe add
WARN_ON_ONCE(pc->flags != PCG_USED);
to check for an unexpected flags usage in the kmem path?
> + memcg = pc->mem_cgroup;
> + pc->flags = 0;
>
> /*
> * We trust that only if there is a memcg associated with the page, it
> --
> 1.9.2
>
--
Michal Hocko
SUSE Labs
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