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From:   Joonsoo Kim <js1304@gmail.com>
Date:   Mon, 23 Mar 2020 13:18:09 +0900
Message-ID: <CAAmzW4Oejc8MFs91v=SWyy-GSLFskcFAGs1-J_4bYKv2Tt=F0g@mail.gmail.com>
Subject: Re: [PATCH v3 1/2] mm/page_alloc: use ac->high_zoneidx for classzone_idx
To:     Baoquan He <bhe@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Minchan Kim <minchan@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>, kernel-team@lge.com,
        Ye Xiaolong <xiaolong.ye@intel.com>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
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2020=EB=85=84 3=EC=9B=94 23=EC=9D=BC (=EC=9B=94) =EC=98=A4=ED=9B=84 12:59, =
Baoquan He <bhe@redhat.com>=EB=8B=98=EC=9D=B4 =EC=9E=91=EC=84=B1:
>
> On 03/23/20 at 12:50pm, Joonsoo Kim wrote:
> > Hello, Baoquan.
> >
> > 2020=EB=85=84 3=EC=9B=94 20=EC=9D=BC (=EA=B8=88) =EC=98=A4=ED=9B=84 7:3=
0, Baoquan He <bhe@redhat.com>=EB=8B=98=EC=9D=B4 =EC=9E=91=EC=84=B1:
> > >
> > >
> > > On 03/20/20 at 05:32pm, js1304@gmail.com wrote:
> > > > From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > > >
> > > > Currently, we use the zone index of preferred_zone which represents
> > > > the best matching zone for allocation, as classzone_idx. It has
> > > > a problem on NUMA systems when the lowmem reserve protection exists
> > > > for some zones on a node that do not exist on other nodes.
> > > >
> > > > In NUMA system, it can be possible that each node has different pop=
ulated
> > > > zones. For example, node 0 could have DMA/DMA32/NORMAL/MOVABLE zone=
 and
> > > > node 1 could have only NORMAL zone. In this setup, allocation reque=
st
> > > > initiated on node 0 and the one on node 1 would have different
> > > > classzone_idx, 3 and 2, respectively, since their preferred_zones a=
re
> > > > different. If the allocation is local, there is no problem. However=
,
> > > > if it is handled by the remote node due to memory shortage, the pro=
blem
> > > > would happen.
> > >
> > > Hi Joonsoo,
> > >
> > > Not sure if adding one sentence into above paragraph would be make it
> > > easier to understand. Assume you are only talking about the high_zone=
idx
> > > is MOVABLE_ZONE with calculation of gfp_zone(gfp_mask), since any oth=
er
> > > case doesn't have this problem. Please correct me if I am wrong.
> >
> > You're right. This example is for the allocation request with
> > gfp_zone(gfp_mask),
> > MOVABLE_ZONE.
> >
> > > In NUMA system, it can be possible that each node has different popul=
ated
> > > zones. For example, node 0 could have DMA/DMA32/NORMAL/MOVABLE zone a=
nd
> > > node 1 could have only NORMAL zone. In this setup, if we get high_zon=
eidx
> > > as 3 (namely MOVABLE zone), with gfp_zone(gfp_mask), allocation reque=
st
> > > initiated on node 0 and the one on node 1 would have different
> > > classzone_idx, 3 and 2, respectively, since their preferred_zones are
> > > different. If the allocation is local, there is no problem. However,
> > > if it is handled by the remote node due to memory shortage, the probl=
em
> > > would happen.
> >
> > I'm okay with your change but I try again to be better. Please check th=
e
> > following rewritten commit message and please let me know if it is bett=
er
> > than before.
>
> Yeah, this new one looks very detailed, I believe anyone interested can
> get what's going on. Thanks for doing this.

Thanks for checking it.
I will submit v4 with including this re-written commit message.

Thanks.