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

2014-08-07 17:53 GMT+09:00 Vlastimil Babka [off-list ref]:

On 08/07/2014 10:19 AM, Joonsoo Kim wrote:

quoted

quoted

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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