On Sat, Dec 31, 2011 at 02:18:43PM +0800, Huang Shijie wrote:

quoted

We get cc->order is -1 when user echos to /proc/sys/vm/compact_memory.
In this case, we should check that if we have enough pages for
the compaction in the zone.

If we do not check this, in our MX6Q board(arm), i ever observed
COMPACT_CLUSTER_MAX pages were compaction failed in per migrate_pages().
That's mean we can not alloc any pages by the free scanner in the zone.

This patch checks the watermark to avoid this problem.

Signed-off-by: Huang Shijie<redacted>
---
  mm/compaction.c |   14 ++++++++++++--
  1 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 899d956..0f12cc9 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c

@@ -442,8 +442,13 @@ static int compact_finished(struct zone *zone,
  	 * order == -1 is expected when compacting via
  	 * /proc/sys/vm/compact_memory
  	 */
-	if (cc->order == -1)
+	if (cc->order == -1) {
+		/* Check if we have enough pages now. */
+		watermark = low_wmark_pages(zone) + COMPACT_CLUSTER_MAX * 2;
+		if (!zone_watermark_ok(zone, 0, watermark, 0, 0))
+			return COMPACT_SKIPPED;
  		return COMPACT_CONTINUE;
+	}

We already do the watermark check in compact_finished. Would moving
the cc->order == -1 check below it not be functionally equivalent? This

Now, I think these code for compact_finish() is not needed. These lines 
can be removed.

The watermark check in the compaction_suitable() is enough to avoid the 
problem.


Huang Shijie

would be preferable to duplicating the code for the watermark check.

Same for the second check in your patch.

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