Date: Thu, 3 Dec 2015 14:37:19 +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
Message-ID: <20151203133719.GF9264@dhcp22.suse.cz>
References: <20151201133455.GB27574@bbox>
 <20151202101643.GC25284@dhcp22.suse.cz>
 <20151203013404.GA30779@bbox>
 <20151203021006.GA31041@bbox>
 <20151203085451.GC9264@dhcp22.suse.cz>
 <20151203125950.GA1428@bbox>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20151203125950.GA1428@bbox>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1832
Lines: 45

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/