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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>