[Devel] [PATCH RHEL7 COMMIT] ms/mm/compaction: change the timing to check to drop the spinlock

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-693.11.6.vz7.42.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-693.11.6.vz7.42.4
------>
commit 1ce03f8056f2be5ab250b46ebc2cc62a670126f0
Author: Joonsoo Kim <iamjoonsoo.kim at lge.com>
Date:   Wed Jan 31 18:21:54 2018 +0300

    ms/mm/compaction: change the timing to check to drop the spinlock
    
    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 at lge.com>
    Acked-by: Vlastimil Babka <vbabka at suse.cz>
    Cc: Mel Gorman <mgorman at suse.de>
    Cc: Rik van Riel <riel at redhat.com>
    Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
    (cherry picked from commit be1aa03b973c7dcdc576f3503f7a60429825c35d)
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 mm/compaction.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index c29883fe146d..c4b6b134b197 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -494,7 +494,7 @@ 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;