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

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Date:   Mon, 28 Aug 2017 09:15:52 +0900
From:   Joonsoo Kim <iamjoonsoo.kim@....com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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
Subject: Re: [PATCH] mm/page_alloc: don't reserve ZONE_HIGHMEM for
 ZONE_MOVABLE request

On Fri, Aug 25, 2017 at 09:38:42AM +0200, Michal Hocko wrote:
> On Fri 25-08-17 09:20:31, Joonsoo Kim wrote:
> > On Thu, Aug 24, 2017 at 11:41:58AM +0200, Vlastimil Babka wrote:
> > > On 08/24/2017 07:45 AM, js1304@...il.com wrote:
> > > > From: Joonsoo Kim <iamjoonsoo.kim@....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.
> > > > 
> > > > And, defining sysctl_lowmem_reserve_ratio array by MAX_NR_ZONES - 1 size
> > > > makes code complex. For example, if there is highmem system, following
> > > > reserve ratio is activated for *NORMAL ZONE* which would be easyily
> > > > misleading people.
> > > > 
> > > >  #ifdef CONFIG_HIGHMEM
> > > >  32
> > > >  #endif
> > > > 
> > > > This patch also fix this situation by defining sysctl_lowmem_reserve_ratio
> > > > array by MAX_NR_ZONES and place "#ifdef" to right place.
> > > > 
> > > > Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
> > > > Acked-by: Vlastimil Babka <vbabka@...e.cz>
> > > 
> > > Looks like I did that almost year ago, so definitely had to refresh my
> > > memory now :)
> > > 
> > > Anyway now I looked more thoroughly and noticed that this change leaks
> > > into the reported sysctl. On a 64bit system with ZONE_MOVABLE:
> > > 
> > > before the patch:
> > > vm.lowmem_reserve_ratio = 256   256     32
> > > 
> > > after the patch:
> > > vm.lowmem_reserve_ratio = 256   256     32      2147483647
> > > 
> > > So if we indeed remove HIGHMEM from protection (c.f. Michal's mail), we
> > > should do that differently than with the INT_MAX trick, IMHO.
> > 
> > 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.
> 
> Why don't we just use 0. In fact we are reserving 0 pages... Using
> INT_MAX is just wrong.

The number of reserved pages is calculated by "managed_pages /
ratio". Using INT_MAX, net result would be 0.

There is a logic converting ratio 0 to ratio 1.

if (sysctl_lowmem_reserve_ratio[idx] < 1)
        sysctl_lowmem_reserve_ratio[idx] = 1

If I use 0 to represent 0 reserved page, there would be a user
who is affected by this change. So, I don't use 0 for this patch.

Thanks.