From: Mel Gorman Date: Wed, 15 Jun 2011 22:08:50 +0000 (-0700) Subject: mm: compaction: ensure that the compaction free scanner does not move to the next... X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=7454f4ba40b419eb999a3c61a99da662bf1a2bb8;p=GitHub%2Fexynos8895%2Fandroid_kernel_samsung_universal8895.git 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 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 the bug should theoritically be possible there. Signed-off-by: Mel Gorman Reviewed-by: Minchan Kim Reviewed-by: Michal Hocko Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/mm/compaction.c b/mm/compaction.c index cf7086c6dc07..9b94eaf537e4 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -144,9 +144,20 @@ static void isolate_freepages(struct zone *zone, int nr_freepages = cc->nr_freepages; struct list_head *freelist = &cc->freepages; + /* + * Initialise the free scanner. The starting point is where we last + * scanned from (or the end of the zone if starting). The low point + * is the end of the pageblock the migration scanner is using. + */ pfn = cc->free_pfn; low_pfn = cc->migrate_pfn + pageblock_nr_pages; - high_pfn = low_pfn; + + /* + * Take care that if the migration scanner is at the end of the zone + * that the free scanner does not accidentally move to the next zone + * in the next isolation cycle. + */ + high_pfn = min(low_pfn, pfn); /* * Isolate free pages until enough are available to migrate the