linux-kernel - Re: [patch] mm: memcontrol: update mem_cgroup_page

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Message-ID: <20141020194634.GA12120@phnom.home.cmpxchg.org>
Date:	Mon, 20 Oct 2014 15:46:34 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch] mm: memcontrol: update mem_cgroup_page_lruvec()
 documentation

On Mon, Oct 20, 2014 at 09:12:56PM +0200, Michal Hocko wrote:
> On Sun 19-10-14 11:30:16, Johannes Weiner wrote:
> > 7512102cf64d ("memcg: fix GPF when cgroup removal races with last
> > exit") added a pc->mem_cgroup reset into mem_cgroup_page_lruvec() to
> > prevent a crash where an anon page gets uncharged on unmap, the memcg
> > is released, and then the final LRU isolation on free dereferences the
> > stale pc->mem_cgroup pointer.
> > 
> > But since 0a31bc97c80c ("mm: memcontrol: rewrite uncharge API"), pages
> > are only uncharged AFTER that final LRU isolation, which guarantees
> > the memcg's lifetime until then.  pc->mem_cgroup now only needs to be
> > reset for swapcache readahead pages.
> 
> Do we want VM_BUG_ON_PAGE(!PageSwapCache, page) into the fixup path?

While that is what we expect as of right now, it's not really a
requirement for this function.  Should somebody later add other page
types they might trigger this assertion and scratch their head about
it and wonder if they're missing some non-obvious dependency.

> > Update the comment and callsite requirements accordingly.
> > 
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> 
> Acked-by: Michal Hocko <mhocko@...e.cz>

Thanks!
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