linux-kernel - Re: memcg uncharge page counter mismatch

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Date:	Thu, 3 Dec 2015 14:37:19 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: memcg uncharge page counter mismatch

On Thu 03-12-15 21:59:50, Minchan Kim wrote:
> On Thu, Dec 03, 2015 at 09:54:52AM +0100, Michal Hocko wrote:
> > On Thu 03-12-15 11:10:06, Minchan Kim wrote:
> > > On Thu, Dec 03, 2015 at 10:34:04AM +0900, Minchan Kim wrote:
> > > > On Wed, Dec 02, 2015 at 11:16:43AM +0100, Michal Hocko wrote:
[...]
> > > > > Also, how big is the underflow?
> > [...]
> > > > nr_pages 293 new -324
> > > > nr_pages 16 new -340
> > > > nr_pages 342 new -91
> > > > nr_pages 246 new -337
> > > > nr_pages 15 new -352
> > > > nr_pages 15 new -367
> > 
> > They are quite large but that is not that surprising if we consider that
> > we are batching many uncharges at once.
> >  
> > > My guess is that it's related to new feature of Kirill's THP 'PageDoubleMap'
> > > so a THP page could be mapped a pte but !pmd_trans_huge(*pmd) so memcg
> > > precharge in move_charge should handle it?
> > 
> > I am not familiar with the current state of THP after the rework
> > unfortunately. So if I got you right then you are saying that
> > pmd_trans_huge_lock fails to notice a THP so we will not charge it as
> > THP and only charge one head page and then the tear down path will
> > correctly recognize it as a THP and uncharge the full size, right?
> 
> Exactly.

Hmm, but are pages represented by those ptes on the LRU list?
__split_huge_pmd_locked doesn't seem to do any lru care. If they are not
on any LRU then mem_cgroup_move_charge_pte_range should ignore such a pte
and the THP (which the pte is part of) should stay in the original
memcg.

How do you trigger this issue btw?
-- 
Michal Hocko
SUSE Labs
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