linux-kernel - Re: [PATCH] mm, memory_hotplug: support movable

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Message-Id: <20170615154320.tzpkjxeuckkua2zm@arbab-laptop.localdomain>
Date:   Thu, 15 Jun 2017 10:43:20 -0500
From:   Reza Arbab <arbab@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...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: [PATCH] mm, memory_hotplug: support movable_node for hotplugable
 nodes

On Thu, Jun 08, 2017 at 02:23:18PM +0200, Michal Hocko wrote:
>movable_node kernel parameter allows to make hotplugable NUMA
>nodes to put all the hotplugable memory into movable zone which
>allows more or less reliable memory hotremove.  At least this
>is the case for the NUMA nodes present during the boot (see
>find_zone_movable_pfns_for_nodes).
>
>This is not the case for the memory hotplug, though.
>
>	echo online > /sys/devices/system/memory/memoryXYZ/status
>
>will default to a kernel zone (usually ZONE_NORMAL) unless the
>particular memblock is already in the movable zone range which is not
>the case normally when onlining the memory from the udev rule context
>for a freshly hotadded NUMA node. The only option currently is to have a
>special udev rule to echo online_movable to all memblocks belonging to
>such a node which is rather clumsy. Not the mention this is inconsistent
>as well because what ended up in the movable zone during the boot will
>end up in a kernel zone after hotremove & hotadd without special care.
>
>It would be nice to reuse memblock_is_hotpluggable but the runtime
>hotplug doesn't have that information available because the boot and
>hotplug paths are not shared and it would be really non trivial to
>make them use the same code path because the runtime hotplug doesn't
>play with the memblock allocator at all.
>
>Teach move_pfn_range that MMOP_ONLINE_KEEP can use the movable zone if
>movable_node is enabled and the range doesn't overlap with the existing
>normal zone. This should provide a reasonable default onlining strategy.
>
>Strictly speaking the semantic is not identical with the boot time
>initialization because find_zone_movable_pfns_for_nodes covers only the
>hotplugable range as described by the BIOS/FW. From my experience this
>is usually a full node though (except for Node0 which is special and
>never goes away completely). If this turns out to be a problem in the
>real life we can tweak the code to store hotplug flag into memblocks
>but let's keep this simple now.
>
>Signed-off-by: Michal Hocko <mhocko@...e.com>

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

-- 
Reza Arbab