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

[Mel Gorman]

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@xxxxxxxxxxxxxxxxxxx>
---
 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