Re: [PATCH] mm, memory_hotplug: support movable_node for hotplugable nodes

[Wei Yang]

On Thu, Jun 08, 2017 at 02:23:18PM +0200, Michal Hocko wrote:
>From: Michal Hocko <mhocko@xxxxxxxx>
>
>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).
>

When movable_node is enabled, we would have overlapped zones, right?
To be specific, only ZONE_MOVABLE could have memory ranges belongs to other
zones.

This looks a little different in the whole ZONE design.

>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
            ^^^

Here is memblock or a memory_block?

>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

So the semantic you want to change here is to make the memory_block in
ZONE_MOVABLE when movable_node is enabled.

Besides this, movable_node is enabled, what other requirements? Like, this
memory_block should next to current ZONE_MOVABLE ? or something else?

>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
                                         ^^^

Hmm... "Not to mentions" looks more understandable.

BTW, I am not a native speaker. If this usage is correct, just ignore this
comment.

>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.
>

Let me try to understand your purpose of this change.

If a memblock has MEMBLOCK_HOTPLU set, it would be in ZONE_MOVABLE during
bootup. While a hotplugged memory_block would be in ZONE_NORMAL without
special care.

So you want to make sure when movable_node is enabled, the hotplugged
memory_block would be in ZONE_MOVABLE. Is this correct?

One more thing is do we have MEMBLOCK_HOTPLU for a hotplugged memory_block?

>Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
>---
>

-- 
Wei Yang
Help you, Help me

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