linux-kernel - Re: [patch 2/2] mm: memcontrol: fix missed end-writeback page accounting

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Message-ID: <20141023141443.GA20526@phnom.home.cmpxchg.org>
Date:	Thu, 23 Oct 2014 10:14:43 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Vladimir Davydov <vdavydov@...allels.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/2] mm: memcontrol: fix missed end-writeback page
 accounting

On Thu, Oct 23, 2014 at 03:03:31PM +0200, Michal Hocko wrote:
> On Wed 22-10-14 14:29:28, Johannes Weiner wrote:
> > 0a31bc97c80c ("mm: memcontrol: rewrite uncharge API") changed page
> > migration to uncharge the old page right away.  The page is locked,
> > unmapped, truncated, and off the LRU, but it could race with writeback
> > ending, which then doesn't unaccount the page properly:
> > 
> > test_clear_page_writeback()              migration
> >   acquire pc->mem_cgroup->move_lock
> 
> I do not think that mentioning move_lock is important/helpful here
> because the hot path which is taken all the time (except when there is a
> task move in progress) doesn't take it.
> Besides that it is not even relevant for the race.

You're right.  It's not worth mentioning the transaction setup/finish
at all, because migration does not participate in that protocol.  How
about this?  Andrew, could you please copy-paste this into the patch?

test_clear_page_writeback()              migration
                                           wait_on_page_writeback()
  TestClearPageWriteback()
                                           mem_cgroup_migrate()
                                             clear PCG_USED
  mem_cgroup_update_page_stat()
    if (PageCgroupUsed(pc))
      decrease memcg pages under writeback
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