[PATCH 3/5] mm/compaction: change the timing to check to drop the spinlock

[Joonsoo Kim]

It is odd to drop the spinlock when we scan (SWAP_CLUSTER_MAX - 1) th pfn
page. This may results in below situation while isolating migratepage.

1. try isolate 0x0 ~ 0x200 pfn pages.
2. When low_pfn is 0x1ff, ((low_pfn+1) % SWAP_CLUSTER_MAX) == 0, so drop
the spinlock.
3. Then, to complete isolating, retry to aquire the lock.

I think that it is better to use SWAP_CLUSTER_MAX th pfn for checking
the criteria about dropping the lock. This has no harm 0x0 pfn, because,
at this time, locked variable would be false.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

diff --git a/mm/compaction.c b/mm/compaction.c
index 0d821a2..b1ba297 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -481,7 +481,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
 	cond_resched();
 	for (; low_pfn < end_pfn; low_pfn++) {
 		/* give a chance to irqs before checking need_resched() */
-		if (locked && !((low_pfn+1) % SWAP_CLUSTER_MAX)) {
+		if (locked && !(low_pfn % SWAP_CLUSTER_MAX)) {
 			if (should_release_lock(&zone->lru_lock)) {
 				spin_unlock_irqrestore(&zone->lru_lock, flags);
 				locked = false;
-- 
1.7.9.5

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