Re: memcg uncharge page counter mismatch

[Minchan Kim <minchan@kernel.org>]

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:
> > > > On Tue 01-12-15 22:34:55, Minchan Kim wrote:
> > > > > With new test on mmotm-2015-11-25-17-08, I saw below WARNING message
> > > > > several times. I couldn't see it with reverting new THP refcount
> > > > > redesign.
> > > > 
> > > > Just a wild guess. What prevents migration/compaction from calling
> > > > split_huge_page on thp zero page? There is VM_BUG_ON but it is not clear
> > > 
> > > I guess migration should work with LRU pages now but zero page couldn't
> > > stay there.
> 
> Ahh, you are right. I have missed PageLRU check in isolate_migratepages_block
> pfn walker.
> 
> > > > whether you run with CONFIG_DEBUG_VM enabled.
> > > 
> > > I enabled VM_DEBUG_VM.
> > > 
> > > > 
> > > > 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.

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