linux-kernel - Re: [RFC PATCH 1/2] mm, memory_hotplug: drop artificial restriction on online/offline

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Message-Id: <20170524215056.h4r3sdk23bn4c2sr@arbab-laptop.localdomain>
Date:   Wed, 24 May 2017 16:50:56 -0500
From:   Reza Arbab <arbab@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        qiuxishi@...wei.com, Kani Toshimitsu <toshi.kani@....com>,
        slaoub@...il.com, Joonsoo Kim <js1304@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [RFC PATCH 1/2] mm, memory_hotplug: drop artificial restriction
 on online/offline

On Wed, May 24, 2017 at 02:24:10PM +0200, Michal Hocko wrote:
>74d42d8fe146 ("memory_hotplug: ensure every online node has NORMAL
>memory") has added can_offline_normal which checks the amount of
>memory in !movable zones as long as CONFIG_MOVABLE_NODE is disable.
>It disallows to offline memory if there is nothing left with a
>justification that "memory-management acts bad when we have nodes which
>is online but don't have any normal memory".
>
>74d42d8fe146 ("memory_hotplug: ensure every online node has NORMAL
>memory") has introduced a restriction that every numa node has to have
>at least some memory in !movable zones before a first movable memory
>can be onlined if !CONFIG_MOVABLE_NODE with the same justification
>
>While it is true that not having _any_ memory for kernel allocations on
>a NUMA node is far from great and such a node would be quite subotimal
>because all kernel allocations will have to fallback to another NUMA
>node but there is no reason to disallow such a configuration in
>principle.
>
>Besides that there is not really a big difference to have one memblock
>for ZONE_NORMAL available or none. With 128MB size memblocks the system
>might trash on the kernel allocations requests anyway. It is really
>hard to draw a line on how much normal memory is really sufficient so
>we have to rely on administrator to configure system sanely therefore
>drop the artificial restriction and remove can_offline_normal and
>can_online_high_movable altogether.

I'm really liking all this cleanup of the memory hotplug code. Thanks!  
Much appreciated.

Acked-by: Reza Arbab <arbab@...ux.vnet.ibm.com>

-- 
Reza Arbab