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authorVlastimil Babka <vbabka@suse.cz>2014-11-13 15:19:30 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2014-11-13 16:17:06 -0800
commit1d5bfe1ffb5b9014ea80bcd8a40ae43a3e213120 (patch)
treecf9efd31dc007cb0c4338c162eda94ebf18e416e
parentdae803e165a11bc88ca8dbc07a11077caf97bbcb (diff)
mm, compaction: prevent infinite loop in compact_zone
Several people have reported occasionally seeing processes stuck in compact_zone(), even triggering soft lockups, in 3.18-rc2+. Testing a revert of commit e14c720efdd7 ("mm, compaction: remember position within pageblock in free pages scanner") fixed the issue, although the stuck processes do not appear to involve the free scanner. Finally, by code inspection, the bug was found in isolate_migratepages() which uses a slightly different condition to detect if the migration and free scanners have met, than compact_finished(). That has not been a problem until commit e14c720efdd7 allowed the free scanner position between individual invocations to be in the middle of a pageblock. In a relatively rare case, the migration scanner position can end up at the beginning of a pageblock, with the free scanner position in the middle of the same pageblock. If it's the migration scanner's turn, isolate_migratepages() exits immediately (without updating the position), while compact_finished() decides to continue compaction, resulting in a potentially infinite loop. The system can recover only if another process creates enough high-order pages to make the watermark checks in compact_finished() pass. This patch fixes the immediate problem by bumping the migration scanner's position to meet the free scanner in isolate_migratepages(), when both are within the same pageblock. This causes compact_finished() to terminate properly. A more robust check in compact_finished() is planned as a cleanup for better future maintainability. Fixes: e14c720efdd73 ("mm, compaction: remember position within pageblock in free pages scanner) Signed-off-by: Vlastimil Babka <vbabka@suse.cz> Reported-by: P. Christeas <xrg@linux.gr> Tested-by: P. Christeas <xrg@linux.gr> Link: http://marc.info/?l=linux-mm&m=141508604232522&w=2 Reported-by: Norbert Preining <preining@logic.at> Tested-by: Norbert Preining <preining@logic.at> Link: https://lkml.org/lkml/2014/11/4/904 Reported-by: Pavel Machek <pavel@ucw.cz> Link: https://lkml.org/lkml/2014/11/7/164 Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Cc: David Rientjes <rientjes@google.com> Cc: Mel Gorman <mel@csn.ul.ie> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--mm/compaction.c8
1 files changed, 6 insertions, 2 deletions
diff --git a/mm/compaction.c b/mm/compaction.c
index 4f0151cfd238..f9792ba3537c 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1039,8 +1039,12 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
}
acct_isolated(zone, cc);
- /* Record where migration scanner will be restarted */
- cc->migrate_pfn = low_pfn;
+ /*
+ * Record where migration scanner will be restarted. If we end up in
+ * the same pageblock as the free scanner, make the scanners fully
+ * meet so that compact_finished() terminates compaction.
+ */
+ cc->migrate_pfn = (end_pfn <= cc->free_pfn) ? low_pfn : cc->free_pfn;
return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
}

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