On Fri, May 23, 2014 at 03:39:38PM +0200, Michal Hocko wrote:
> 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>
Thanks!
> > 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?
That makes sense, I added the following:
+ /*
+ * The page is freshly allocated and not visible to any
+ * outside callers yet. Set up pc non-atomically.
+ */
> > 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?
There is no overlap between page types that use PCG_USED and those
that don't. What would be the value of adding this?
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