linux-kernel - Re: [Resend PATCH 1/6] mm/memcg: warning on !memcg after readahead page charged

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Message-ID: <20200821134842.GF32537@dhcp22.suse.cz>
Date:   Fri, 21 Aug 2020 15:48:42 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Qian Cai <cai@....pw>
Cc:     Alex Shi <alex.shi@...ux.alibaba.com>, akpm@...ux-foundation.org,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, nao.horiguchi@...il.com,
        osalvador@...e.de, mike.kravetz@...cle.com
Subject: Re: [Resend PATCH 1/6] mm/memcg: warning on !memcg after readahead
 page charged

On Fri 21-08-20 08:39:37, Qian Cai wrote:
> On Fri, Aug 21, 2020 at 10:01:27AM +0200, Michal Hocko wrote:
> > On Thu 20-08-20 10:58:51, Qian Cai wrote:
> > > On Tue, Aug 11, 2020 at 07:10:27PM +0800, Alex Shi wrote:
> > > > Since readahead page is charged on memcg too, in theory we don't have to
> > > > check this exception now. Before safely remove them all, add a warning
> > > > for the unexpected !memcg.
> > > > 
> > > > Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com>
> > > > Acked-by: Michal Hocko <mhocko@...e.com>
> > > 
> > > This will trigger,
> > 
> > Thanks for the report!
> >  
> > > [ 1863.916499] LTP: starting move_pages12
> > > [ 1863.946520] page:000000008ccc1062 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x1fd3c0
> > > [ 1863.946553] head:000000008ccc1062 order:5 compound_mapcount:0 compound_pincount:0
> > 
> > Hmm, this is really unexpected. How did we get order-5 page here? Is
> > this some special mappaing that sys_move_pages should just ignore?
> 
> Well, I thought everybody should be able to figure out where to find the LTP
> tests source code at this stage to see what it does. Anyway, the test simply
> migrate hugepages while soft offlining, so order 5 is expected as that is 2M
> hugepage on powerpc (also reproduced on x86 below). It might be easier to
> reproduce using our linux-mm random bug collection on NUMA systems.

OK, I must have missed that this was on ppc. The order makes more sense
now. I will have a look at this next week.

Thanks!
-- 
Michal Hocko
SUSE Labs