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Subject: Re: [PATCH] mm/page_alloc: don't reserve ZONE_HIGHMEM for
 ZONE_MOVABLE request
References: <1503553546-27450-1-git-send-email-iamjoonsoo.kim@lge.com>
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From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <ad269513-56e4-87af-f44d-86a5dba1c9f6@suse.cz>
Date: Tue, 29 Aug 2017 09:00:16 +0200
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To: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Michal Hocko <mhocko@suse.com>, 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, Linux API <linux-api@vger.kernel.org>

On 08/29/2017 02:36 AM, Joonsoo Kim wrote:
> On Mon, Aug 28, 2017 at 08:45:07AM +0200, Vlastimil Babka wrote:
>> +CC linux-api
>>
>> On 08/28/2017 02:28 AM, Joonsoo Kim wrote:
>>> On Fri, Aug 25, 2017 at 09:56:10AM +0200, Vlastimil Babka wrote:
>>>
>>> Seems reasonable. However, if there is a user who checks
>>> sysctl_lowmem_reserve_ratio entry for HIGHMEM and change it, suggested
>>> interface will cause a problem since it doesn't expose ratio for
>>> HIGHMEM. Am I missing something?
>>
>> As you explained, it makes little sense to change it for HIGHMEM which
>> only affects MOVABLE allocations. Also I doubt there are many systems
>> with both HIGHMEM (implies 32bit) *and* MOVABLE (implies NUMA, memory
>> hotplug...) zones. So I would just remove it, and if somebody will
>> really miss it, we can always add it back. In any case, please CC
>> linux-api on the next version.
> 
> If we will accept a change that potentially breaks the user, I think
> that making zero as a special value for sysctl_lowmem_reserve_ratio
> is better solution. How about this way?

I'd prefer removal, but won't object to zero. Certainly much better than
UINT_MAX.

> Thanks.
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