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

[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.

Vlastimil



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