linux-kernel - Re: [PATCH] mm/page_alloc: don't reserve ZONE_HIGHMEM for ZONE

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Date:   Mon, 28 Aug 2017 08:45:07 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Johannes Weiner <hannes@...xchg.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] mm/page_alloc: don't reserve ZONE_HIGHMEM for
 ZONE_MOVABLE request

+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:
>> On 08/25/2017 02:20 AM, Joonsoo Kim wrote:
>>> On Thu, Aug 24, 2017 at 11:41:58AM +0200, Vlastimil Babka wrote:
>>>
>>> Hmm, this is already pointed by Minchan and I have answered that.
>>>
>>> lkml.kernel.org/r/<20170421013243.GA13966@...304-desktop>
>>>
>>> If you have a better idea, please let me know.
>>
>> My idea is that size of sysctl_lowmem_reserve_ratio is ZONE_NORMAL+1 and
>> it has no entries for zones > NORMAL. The
>> setup_per_zone_lowmem_reserve() is adjusted to only set
>> lower_zone->lowmem_reserve[j] for idx <= ZONE_NORMAL.
>>
>> I can't imagine somebody would want override the ratio for HIGHMEM or
>> MOVABLE
>> (where it has no effect anyway) so the simplest thing is not to expose
>> it at all.
> 
> 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.

> Thanks.
> 
> 
>>
>>> Thanks.
>>>
>>
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