On Thu, Aug 09, 2012 at 09:12:12AM +0900, Minchan Kim wrote:
> > <SNIP>
> >
> > Second, it updates compact_cached_free_pfn in a more limited set of
> > circumstances.
> >
> > If a scanner has wrapped, it updates compact_cached_free_pfn to the end
> > of the zone. When a wrapped scanner isolates a page, it updates
> > compact_cached_free_pfn to point to the highest pageblock it
> > can isolate pages from.
>
> Okay until here.
>
Great.
> >
> > If a scanner has not wrapped when it has finished isolated pages it
> > checks if compact_cached_free_pfn is pointing to the end of the
> > zone. If so, the value is updated to point to the highest
> > pageblock that pages were isolated from. This value will not
> > be updated again until a free page scanner wraps and resets
> > compact_cached_free_pfn.
>
> I tried to understand your intention of this part but unfortunately failed.
> By this part, the problem you mentioned could happen again?
>
Potentially yes, I did say it still races in the changelog.
> C
> Process A M S F
> |---------------------------------------|
> Process B M FS
>
> C is zone->compact_cached_free_pfn
> S is cc->start_pfree_pfn
> M is cc->migrate_pfn
> F is cc->free_pfn
>
> In this diagram, Process A has just reached its migrate scanner, wrapped
> around and updated compact_cached_free_pfn to end of the zone accordingly.
>
Yes. Now that it has wrapped it updates the compact_cached_free_pfn
every loop of isolate_freepages here.
if (isolated) {
high_pfn = max(high_pfn, pfn);
/*
* If the free scanner has wrapped, update
* compact_cached_free_pfn to point to the highest
* pageblock with free pages. This reduces excessive
* scanning of full pageblocks near the end of the
* zone
*/
if (cc->order > 0 && cc->wrapped)
zone->compact_cached_free_pfn = high_pfn;
}
> Simultaneously, Process B finishes isolating in a block and peek
> compact_cached_free_pfn position and know it's end of the zone so
> update compact_cached_free_pfn to highest pageblock that pages were
> isolated from.
>
Yes, they race at this point. One of two things happen here and I agree
that this is racy
1. Process A does another iteration of its loop and sets it back
2. Process A does not do another iteration of the loop, the cached_pfn
is further along that it should. The next compacting process will
wrap early and reset cached_pfn again but continue to scan the zone.
Either option is relatively harmless because in both cases the zone gets
scanned. In patch 4 it was possible that large portions of the zone were
frequently missed.
> Process A updates compact_cached_free_pfn to the highest pageblock which
> was set by process B because process A has wrapped. It ends up big jump
> without any scanning in process A.
>
It recovers quickly and is nowhere near as severe as what patch 4
suffers from.
--
Mel Gorman
SUSE Labs
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