[Devel] [PATCH RHEL7 COMMIT] ms/mm, compaction: avoid isolating pinned pages

[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 705c7b021d36a69380442e37421e5164212cadaa
Author: David Rientjes <rientjes at google.com>
Date:   Wed Jan 31 18:21:53 2018 +0300

    ms/mm, compaction: avoid isolating pinned pages
    
    Page migration will fail for memory that is pinned in memory with, for
    example, get_user_pages().  In this case, it is unnecessary to take
    zone->lru_lock or isolating the page and passing it to page migration
    which will ultimately fail.
    
    This is a racy check, the page can still change from under us, but in
    that case we'll just fail later when attempting to move the page.
    
    This avoids very expensive memory compaction when faulting transparent
    hugepages after pinning a lot of memory with a Mellanox driver.
    
    On a 128GB machine and pinning ~120GB of memory, before this patch we
    see the enormous disparity in the number of page migration failures
    because of the pinning (from /proc/vmstat):
    
            compact_pages_moved 8450
            compact_pagemigrate_failed 15614415
    
    0.05% of pages isolated are successfully migrated and explicitly
    triggering memory compaction takes 102 seconds.  After the patch:
    
            compact_pages_moved 9197
            compact_pagemigrate_failed 7
    
    99.9% of pages isolated are now successfully migrated in this
    configuration and memory compaction takes less than one second.
    
    Signed-off-by: David Rientjes <rientjes at google.com>
    Acked-by: Hugh Dickins <hughd at google.com>
    Acked-by: Mel Gorman <mgorman at suse.de>
    Cc: Joonsoo Kim <iamjoonsoo.kim at lge.com>
    Cc: Rik van Riel <riel at redhat.com>
    Cc: Greg Thelen <gthelen at google.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 119d6d59dcc0980dcd581fdadb6b2033b512a473)
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 mm/compaction.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/compaction.c b/mm/compaction.c
index bbb1f65b0041..981e6b601bfe 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -588,6 +588,15 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
 			continue;
 		}
 
+		/*
+		 * Migration will fail if an anonymous page is pinned in memory,
+		 * so avoid taking lru_lock and isolating it unnecessarily in an
+		 * admittedly racy check.
+		 */
+		if (!page_mapping(page) &&
+		    page_count(page) > page_mapcount(page))
+			continue;
+
 		/* Check if it is ok to still hold the lock */
 		locked = compact_checklock_irqsave(&zone->lru_lock, &flags,
 								locked, cc);