linux-kernel - Re: [PATCH v3 1/2] mm/page_alloc: use ac->high_zoneidx for classzone

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Message-ID: <CAAmzW4Oejc8MFs91v=SWyy-GSLFskcFAGs1-J_4bYKv2Tt=F0g@mail.gmail.com>
Date:   Mon, 23 Mar 2020 13:18:09 +0900
From:   Joonsoo Kim <js1304@...il.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Minchan Kim <minchan@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>, kernel-team@....com,
        Ye Xiaolong <xiaolong.ye@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v3 1/2] mm/page_alloc: use ac->high_zoneidx for classzone_idx

2020년 3월 23일 (월) 오후 12:59, Baoquan He <bhe@...hat.com>님이 작성:
>
> On 03/23/20 at 12:50pm, Joonsoo Kim wrote:
> > Hello, Baoquan.
> >
> > 2020년 3월 20일 (금) 오후 7:30, Baoquan He <bhe@...hat.com>님이 작성:
> > >
> > >
> > > On 03/20/20 at 05:32pm, js1304@...il.com wrote:
> > > > From: Joonsoo Kim <iamjoonsoo.kim@....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 populated
> > > > zones. For example, node 0 could have DMA/DMA32/NORMAL/MOVABLE zone and
> > > > node 1 could have only NORMAL zone. In this setup, allocation request
> > > > 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 problem
> > > > 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_zoneidx
> > > is MOVABLE_ZONE with calculation of gfp_zone(gfp_mask), since any other
> > > 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 populated
> > > zones. For example, node 0 could have DMA/DMA32/NORMAL/MOVABLE zone and
> > > node 1 could have only NORMAL zone. In this setup, if we get high_zoneidx
> > > as 3 (namely MOVABLE zone), with gfp_zone(gfp_mask), allocation request
> > > 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 problem
> > > would happen.
> >
> > I'm okay with your change but I try again to be better. Please check the
> > following rewritten commit message and please let me know if it is better
> > 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.