Re: [PATCH v2 5/8] mm/isolation: change pageblock isolation logic to fix freepage counting bugs

[Joonsoo Kim <js1304@gmail.com>]

2014-08-07 17:53 GMT+09:00 Vlastimil Babka <vbabka@suse.cz>:
> On 08/07/2014 10:19 AM, Joonsoo Kim wrote:
>>>
>>> Is it needed to disable the pcp list? Shouldn't drain be enough?
>>> After the drain you already are sure that future freeing will see
>>> MIGRATE_ISOLATE and skip pcp list anyway, so why disable it
>>> completely?
>>
>>
>> Yes, it is needed. Until we move freepages from normal buddy list
>> to isolate buddy list, freepages could be allocated by others. In this
>> case, they could be moved to pcp list. When it is flushed from pcp list
>> to buddy list, we need to check whether it is on isolate migratetype
>> pageblock or not. But, we don't want that hook in free_pcppages_bulk()
>> because it is page allocator's normal freepath. To remove it, we shoule
>> disable the pcp list here.
>
>
> Ah, right. I thought that everything going to pcp lists would be through
> freeing which would already observe the isolate migratetype and skip
> pcplist. I forgot about the direct filling of pcplists from buddy list.
> You're right that we don't want extra hooks there.
>
> Still, couldn't this be solved in a simpler way via another pcplist drain
> after the pages are moved from normal to isolate buddy list? Should be even
> faster because instead of disable - drain - enable (5 all-cpu kicks, since
> each pageset_update does 2 kicks) you have drain - drain (2 kicks). While
> it's true that pageset_update is single-zone operation, I guess we would
> easily benefit from having a single-zone drain operation as well.

I hope so, but, it's not possible. Consider following situation.

Page A: on pcplist of CPU2 and it is on isolate pageblock.

CPU 1                   CPU 2
drain pcplist
wait IPI finished     move A to normal buddy list
finish IPI
                            A is moved to pcplist by allocation request

move doesn't catch A,
because it is on pcplist.

drain pcplist
wait IPI finished     move A to normal buddy list
finish IPI
                            A is moved to pcplist by allocation request

repeat!!

It could happen infinitely, though, low possibility.

Thanks.

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