Date: Fri, 25 Aug 2017 09:15:43 +0900
From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>,
        Minchan Kim <minchan@kernel.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/page_alloc: don't reserve ZONE_HIGHMEM for
 ZONE_MOVABLE request
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 <20170824093050.GD5943@dhcp22.suse.cz>
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On Thu, Aug 24, 2017 at 11:30:50AM +0200, Michal Hocko wrote:
> On Thu 24-08-17 14:45:46, Joonsoo Kim wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > 
> > Freepage on ZONE_HIGHMEM doesn't work for kernel memory so it's not that
> > important to reserve. When ZONE_MOVABLE is used, this problem would
> > theorectically cause to decrease usable memory for GFP_HIGHUSER_MOVABLE
> > allocation request which is mainly used for page cache and anon page
> > allocation. So, fix it.
> 
> I do not really understand what is the problem you are trying to fix.
> Yes the memory is reserved for a higher priority consumer and that is
> deliberate AFAICT. Just consider that an OOM victim wants to make
> further progress and rely on memory reserve while doing
> GFP_HIGHUSER_MOVABLE request.
> 
> So what is the real problem you are trying to address here?

If the system has the both, ZONE_HIGHMEM and ZONE_MOVABLE,
ZONE_HIGHMEM will reserve the memory for ZONE_MOVABLE request.
However, they are consumed by nearly equivalent priority consumer who
uses GFP_HIGHMEM + GFP_MOVABLE. In that case, reserved memory in
ZONE_HIGHMEM would not be used and it means just waste of the memory.
This patch try to fix it to nullify reserving memory in ZONE_HIGHMEM.

And, I think that all this problem is caused by the complex code in
lowmem reserve calculation. So, did some clean-up.

Thanks.