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

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Date:   Fri, 21 Aug 2020 10:01:43 -0400
From:   Qian Cai <cai@....pw>
To:     Michal Hocko <mhocko@...e.com>
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, Aug 21, 2020 at 03:48:42PM +0200, Michal Hocko wrote:
> 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.

Sorry about not mentioning powerpc in the first place. I don't know since when
powerpc will no longer print out hardware information like x86 does in those
warning reports. I'll dig.