Date: Fri, 4 Dec 2015 10:58:15 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Minchan Kim <minchan@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Johannes Weiner <hannes@cmpxchg.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: memcg uncharge page counter mismatch
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On Fri 04-12-15 18:16:34, Minchan Kim wrote:
> On Fri, Dec 04, 2015 at 09:52:27AM +0100, Michal Hocko wrote:
> > On Fri 04-12-15 14:35:15, Minchan Kim wrote:
> > > On Thu, Dec 03, 2015 at 04:47:29PM +0100, Michal Hocko wrote:
> > > > On Thu 03-12-15 15:58:50, Michal Hocko wrote:
> > > > [....]
> > > > > Warning, this looks ugly as hell.
> > > > 
> > > > I was thinking about it some more and it seems that we should rather not
> > > > bother with partial thp at all and keep it in the original memcg
> > > > instead. It is way much less code and I do not think this will be too
> > > > disruptive. Somebody should be holding the thp head, right?
> > > > 
> > > > Minchan, does this fix the issue you are seeing.
> > > 
> > > This patch solves the issue but not sure it's right approach.
> > > I think it could make regression that in old, we could charge
> > > a THP page but we can't now.
> > 
> > The page would still get charged when allocated. It just wouldn't get
> > moved when mapped only partially. IIUC there will be still somebody
> > mapping the THP head via pmd, right? That process will move the page to
> 
> If I read code correctly, No. The split_huge_pmd splits just pmd,
> not page itself. IOW, it could be possible !pmd_trans_huge(pmd) &&
> PageTransHuge although there is only process owns the page.

I am not sure I follow you. I thought there would still be other pmd
which will hold the THP. Why should we keep the page as huge when all
processes which map it have already split it up?

On the other hand it is true that the last process which maps the whole
thp might have exited and leave others to map it partially.
 
> > the new memcg when moved. Or is it possible that we will end up only
> > with pte mapped THP from all processes? Kirill?
> 
> I'm not Kirill but I think it's possible.
> If so, a thing we can use is page_mapcount(page) == 1. With that,
> it could gaurantee only a process owns the page so charge 512 instead of 1?

Alright the exclusive holder should indeed move it. I will think how to
simplify the previous patch (has it helped in your testing btw.?).

-- 
Michal Hocko
SUSE Labs
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