Re: memcg uncharge page counter mismatch

From: Minchan Kim <minchan@kernel.org>
To: Michal Hocko <mhocko@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
Date: Fri, 4 Dec 2015 18:16:34 +0900	[thread overview]
Message-ID: <20151204091634.GB5174@blaptop> (raw)
In-Reply-To: <20151204085226.GB10021@dhcp22.suse.cz>

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.

> 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?

> 
> If not then I think it is reasonable to expect that partially mapped THP
> is not moved during task migration. I will post an official patch after
> Kirill confirms my understanding.
> 
> Anyway thanks for the testing and pointing me to right direction
> Minchan!

Thanks for the quick patch and feedback, Michal.

> 
> -- 
> Michal Hocko
> SUSE Labs

-- 
Kind regards,
Minchan Kim

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