Re: [PATCH 0/3] mm: fixup changers of per cpu pageset's ->high and ->batch

[KOSAKI Motohiro <kosaki.motohiro@gmail.com>]

(4/8/13 1:16 PM), Cody P Schafer wrote:
> On 04/07/2013 08:23 AM, KOSAKI Motohiro wrote:
>> (4/5/13 4:33 PM), Cody P Schafer wrote:
>>> In one case while modifying the ->high and ->batch fields of per cpu pagesets
>>> we're unneededly using stop_machine() (patches 1 & 2), and in another we don't have any
>>> syncronization at all (patch 3).
>>>
>>> This patchset fixes both of them.
>>>
>>> Note that it results in a change to the behavior of zone_pcp_update(), which is
>>> used by memory_hotplug. I _think_ that I've diserned (and preserved) the
>>> essential behavior (changing ->high and ->batch), and only eliminated unneeded
>>> actions (draining the per cpu pages), but this may not be the case.
>>
>> at least, memory hotplug need to drain.
> 
> Could you explain why the drain is required here? From what I can tell,
> after the stop_machine() completes, the per cpu page sets could be
> repopulated at any point, making the combination of draining and
> modifying ->batch & ->high uneeded.

Then, memory hotplug again and again try to drain. Moreover hotplug prevent repopulation
by using MIGRATE_ISOLATE.
pcp never be page count == 0 and it prevent memory hot remove.

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