linux-kernel - Re: [RFC PATCH] mm/vmscan: remove two un-needed mem_cgroup_page

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Date:	Sat, 15 Feb 2014 21:25:39 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Weijie Yang <weijie.yang.kh@...il.com>
cc:	Hugh Dickins <hughd@...gle.com>,
	Weijie Yang <weijie.yang@...sung.com>, riel@...hat.com,
	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [RFC PATCH] mm/vmscan: remove two un-needed mem_cgroup_page_lruvec()
 call

On Sun, 16 Feb 2014, Weijie Yang wrote:
> On Sun, Feb 16, 2014 at 12:00 PM, Hugh Dickins <hughd@...gle.com> wrote:
> > On Sun, 16 Feb 2014, Weijie Yang wrote:
> >
> >> In putback_inactive_pages() and move_active_pages_to_lru(),
> >> lruvec is already an input parameter and pages are all from this lruvec,
> >> therefore there is no need to call mem_cgroup_page_lruvec() in loop.
> >>
> >> Signed-off-by: Weijie Yang <weijie.yang@...sung.com>
> >
> > Looks plausible but I believe it's incorrect.  The lruvec passed in
> > is the one we took the pages from, but there's a small but real chance
> > that the page has become uncharged meanwhile, and should now be put back
> > on the root_mem_cgroup's lruvec instead of the original memcg's lruvec.
> 
> Hi Hugh,
> 
> Thanks for your review.
> Frankly speaking, I am not very sure about it, that is why I add a RFC tag here.
> So,  do we need update the reclaim_stat meanwhile as we change the lruvec?

No, it's not worth bothering about, it's only for stats and this is an
unlikely case; whereas wrong memcg can be a significant correctness issue.

Hugh
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