Re: [PATCH] mm: compaction: Abort compaction if too many pages areisolated and caller is asynchronous

[Minchan Kim]

On Fri, Jun 03, 2011 at 03:49:41PM +0100, Mel Gorman wrote:
> On Fri, Jun 03, 2011 at 03:09:20AM +0100, Mel Gorman wrote:
> > On Thu, Jun 02, 2011 at 05:37:54PM +0200, Andrea Arcangeli wrote:
> > > > There is an explanation in here somewhere because as I write this,
> > > > the test machine has survived 14 hours under continual stress without
> > > > the isolated counters going negative with over 128 million pages
> > > > successfully migrated and a million pages failed to migrate due to
> > > > direct compaction being called 80,000 times. It's possible it's a
> > > > co-incidence but it's some co-incidence!
> > > 
> > > No idea...
> > 
> > I wasn't able to work on this most of the day but was looking at this
> > closer this evening again and I think I might have thought of another
> > theory that could cause this problem.
> > 
> > When THP is isolating pages, it accounts for the pages isolated against
> > the zone of course. If it backs out, it finds the pages from the PTEs.
> > On !SMP but PREEMPT, we may not have adequate protection against a new
> > page from a different zone being inserted into the PTE causing us to
> > decrement against the wrong zone. While the global counter is fine,
> > the per-zone counters look corrupted. You'd still think it was the
> > anon counter tht got screwed rather than the file one if it really was
> > THP unfortunately so it's not the full picture. I'm going to start
> > a test monitoring both zoneinfo and vmstat to see if vmstat looks
> > fine while the per-zone counters that are negative are offset by a
> > positive count on the other zones that when added together become 0.
> > Hopefully it'll actually trigger overnight :/
> > 
> 
> Right idea of the wrong zone being accounted for but wrong place. I
> think the following patch should fix the problem;
> 
> ==== CUT HERE ===
> mm: compaction: Ensure that the compaction free scanner does not move to the next zone
> 
> Compaction works with two scanners, a migration and a free
> scanner. When the scanners crossover, migration within the zone is
> complete. The location of the scanner is recorded on each cycle to
> avoid excesive scanning.
> 
> When a zone is small and mostly reserved, it's very easy for the
> migration scanner to be close to the end of the zone. Then the following
> situation can occurs
> 
>   o migration scanner isolates some pages near the end of the zone
>   o free scanner starts at the end of the zone but finds that the
>     migration scanner is already there
>   o free scanner gets reinitialised for the next cycle as
>     cc->migrate_pfn + pageblock_nr_pages
>     moving the free scanner into the next zone
>   o migration scanner moves into the next zone but continues accounting
>     against the old zone
> 
> When this happens, NR_ISOLATED accounting goes haywire because some
> of the accounting happens against the wrong zone. One zones counter
> remains positive while the other goes negative even though the overall
> global count is accurate. This was reported on X86-32 with !SMP because
> !SMP allows the negative counters to be visible. The fact that it is
> difficult to reproduce on X86-64 is probably just a co-incidence as

I guess it's related to zone sizes.
X86-64 has small DMA and large DMA32 zones for fallback of NORMAL while
x86 has just a small DMA(16M) zone.

I think DMA zone in x86 is easily full of non-LRU or non-movable pages.
So isolate_migratepagse continues to scan for finding pages which are migratable
and then it reaches near end of zone.


> the bug should theoritically be possible there.
> Finally, you found it. Congratulations on!
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>

When we are debugging this problem, we found a few of bugs and enhance points
and submitted patches. It was a very good chance to fix Linux VM.

Thanks, Mel.

-- 
Kind regards
Minchan Kim
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