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SubjectRe: [PATCH 1/4] mm: compaction: Ensure that the compaction free scanner does not move to the next zone
On Tue 07-06-11 16:07:02, Mel Gorman wrote:
> 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
> 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
> the bug should theoritically be possible there.
> Signed-off-by: Mel Gorman <>
> Reviewed-by: Minchan Kim <>

Reviewed-by: Michal Hocko <>

Michal Hocko
Lihovarska 1060/12
190 00 Praha 9
Czech Republic

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