The patch titled
mm: compaction: ensure that the compaction free scanner does not move to the next zone
has been added to the -mm tree. Its filename is
mm-compaction-ensure-that-the-compaction-free-scanner-does-not-move-to-the-next-zone.patch
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Subject: mm: compaction: ensure that the compaction free scanner does not move to the next zone
From: Mel Gorman <mgorman@xxxxxxx>
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 <mgorman@xxxxxxx>
Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>
Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/compaction.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
diff -puN mm/compaction.c~mm-compaction-ensure-that-the-compaction-free-scanner-does-not-move-to-the-next-zone mm/compaction.c
--- a/mm/compaction.c~mm-compaction-ensure-that-the-compaction-free-scanner-does-not-move-to-the-next-zone
+++ a/mm/compaction.c
@@ -144,9 +144,20 @@ static void isolate_freepages(struct zon
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
_
Patches currently in -mm which might be from mgorman@xxxxxxx are
mm-compaction-fix-special-case-1-order-checks.patch
mm-migratec-dont-account-swapcache-as-shmem.patch
mm-compaction-ensure-that-the-compaction-free-scanner-does-not-move-to-the-next-zone.patch
mm-vmscan-do-not-use-page_count-without-a-page-pin.patch
mm-compaction-abort-compaction-if-too-many-pages-are-isolated-and-caller-is-asynchronous-v2.patch
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