[PATCH 06/14] mm, migrate: Immediately fail migration of a page with no migration handler

Mel Gorman
2018-12-14T23:03:47Z

Pages with no migration handler use a fallback hander which sometimes
works and sometimes persistently fails such as blockdev pages. Migration
will retry a number of times on these persistent pages which is wasteful
during compaction. This patch will fail migration immediately unless the
caller is in MIGRATE_SYNC mode which indicates the caller is willing to
wait while being persistent.

This is not expected to help THP allocation success rates but it does
reduce latencies slightly.

1-socket thpfioscale
4.20.0-rc6 4.20.0-rc6
noreserved-v1r4 failfast-v1r4
Amean fault-both-1 0.00 ( 0.00%) 0.00 * 0.00%*
Amean fault-both-3 2276.15 ( 0.00%) 3867.54 * -69.92%*
Amean fault-both-5 4992.20 ( 0.00%) 5313.20 ( -6.43%)
Amean fault-both-7 7373.30 ( 0.00%) 7039.11 ( 4.53%)
Amean fault-both-12 11911.52 ( 0.00%) 11328.29 ( 4.90%)
Amean fault-both-18 17209.42 ( 0.00%) 16455.34 ( 4.38%)
Amean fault-both-24 20943.71 ( 0.00%) 20448.94 ( 2.36%)
Amean fault-both-30 22703.00 ( 0.00%) 21655.07 ( 4.62%)
Amean fault-both-32 22461.41 ( 0.00%) 21415.35 ( 4.66%)

The 2-socket results are not materially different. Scan rates are
similar as expected.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
mm/migrate.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index df17a710e2c7..0e27a10429e2 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -885,7 +885,7 @@ static int fallback_migrate_page(struct address_space *mapping,
*/
if (page_has_private(page) &&
!try_to_release_page(page, GFP_KERNEL))
- return -EAGAIN;
+ return mode == MIGRATE_SYNC ? -EAGAIN : -EBUSY;

return migrate_page(mapping, newpage, page, mode);
}
--
2.16.4