Re: [PATCH -v2 4/6] memcg: make sure that memcg is not offline when charging

[Michal Hocko <mhocko@suse.cz>]

On Wed 05-02-14 11:45:43, Johannes Weiner wrote:
> On Wed, Feb 05, 2014 at 05:19:40PM +0100, Michal Hocko wrote:
> > On Wed 05-02-14 10:28:21, Johannes Weiner wrote:
> > > Here is the only exception to the above: swapout records maintain
> > > permanent css references, so they prevent css_free() from running.
> > > For that reason alone we should run one optimistic reparenting in
> > > css_offline() to make sure one swap record does not pin gigabytes of
> > > pages in an offlined cgroup, which is unreachable for reclaim.  But
> > 
> > How can reparenting help for swapped out pages? Or did you mean to at
> > least get rid of swapcache pages?
> 
> I was thinking primarily of page cache.  There could be a lot of it
> left in the group and once css_tryget() is disabled we can't reclaim
> it anymore.

Good point.

> So we'd clean that out at offline time optimistically and
> at css_free() we catch any charges raced that showed up afterwards.

OK, care to send a patch which would clarify the reparenting usage in
both css_offline and css_free?

-- 
Michal Hocko
SUSE Labs

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