Re: [PATCH 06/11] mm, compaction: distinguish between full and partial COMPACT_COMPLETE

[Michal Hocko <mhocko@kernel.org>]

On Mon 11-04-16 14:53:36, Vlastimil Babka wrote:
> On 04/11/2016 02:46 PM, Michal Hocko wrote:
> >>This assumes that migrate scanner at initial position implies also free
> >>scanner at the initial position. That should be true, because migration
> >>scanner is the first to run. But getting the zone->compact_cached_*_pfn is
> >>racy. Worse, zone->compact_cached_migrate_pfn is array distinguishing sync
> >>and async compaction, so it's possible that async compaction has advanced
> >>both its own migrate scanner cached position, and the shared free scanner
> >>cached position, and then sync compaction starts migrate scanner at
> >>start_pfn, but free scanner has already advanced.
> >
> >OK, I see. The whole thing smelled racy but I thought it wouldn't be
> >such a big deal. Even if we raced then only a marginal part of the zone
> >wouldn't be scanned, right? Or is it possible that free_pfn would appear
> >in the middle of the zone because of the race?
> 
> The racy part is negligible but I didn't realize the sync/async migrate
> scanner part until now. So yeah, free_pfn could have got to middle of zone
> when it was in the async mode. But that also means that the async mode
> recently used up all free pages in the second half of the zone. WRT free
> pages isolation, async mode is not trying less than sync, so it shouldn't be
> a considerable missed opportunity if we don't rescan the it, though.

I am not really sure I understand. The primary intention of this patch
is to distinguish where we have scanned basically whole zones from cases
where a new scan started off previous mark and so it was just unlucky to
see only tiny bit of the zone where we would clearly give up too early.
FWIU this shouldn't be the case if we start scanning from the beginning
of the zone even if we raced on the other end of the zone because the
missed part would be negligible. Is that understanding correct?
-- 
Michal Hocko
SUSE Labs

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