Re: memcg uncharge page counter mismatch

[Michal Hocko <mhocko@kernel.org>]

On Thu 03-12-15 14:37:19, Michal Hocko wrote:
> 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.

Ohh, PageLRU is
PAGEFLAG(LRU, lru, PF_HEAD)

So we are checking the head and it is on LRU. Now I can see how this
might happen. Let me think about a fix...
-- 
Michal Hocko
SUSE Labs