[PATCH v4 2/4] mm: failfast mode with __GFP_NORETRY in alloc_contig_range

[Minchan Kim <minchan@kernel.org>]

Contiguous memory allocation can be stalled due to waiting
on page writeback and/or page lock which causes unpredictable
delay. It's a unavoidable cost for the requestor to get *big*
contiguous memory but it's expensive for *small* contiguous
memory(e.g., order-4) because caller could retry the request
in different range where would have easy migratable pages
without stalling.

This patch introduce __GFP_NORETRY as compaction gfp_mask in
alloc_contig_range so it will fail fast without blocking
when it encounters pages needed waiting.

Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 mm/page_alloc.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b031a5ae0bd5..1cdc3ee0b22e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8491,12 +8491,16 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
 	unsigned int nr_reclaimed;
 	unsigned long pfn = start;
 	unsigned int tries = 0;
+	unsigned int max_tries = 5;
 	int ret = 0;
 	struct migration_target_control mtc = {
 		.nid = zone_to_nid(cc->zone),
 		.gfp_mask = GFP_USER | __GFP_MOVABLE | __GFP_RETRY_MAYFAIL,
 	};
 
+	if (cc->alloc_contig && cc->mode == MIGRATE_ASYNC)
+		max_tries = 1;
+
 	migrate_prep();
 
 	while (pfn < end || !list_empty(&cc->migratepages)) {
@@ -8513,7 +8517,7 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
 				break;
 			}
 			tries = 0;
-		} else if (++tries == 5) {
+		} else if (++tries == max_tries) {
 			ret = ret < 0 ? ret : -EBUSY;
 			break;
 		}
@@ -8564,7 +8568,7 @@ int alloc_contig_range(unsigned long start, unsigned long end,
 		.nr_migratepages = 0,
 		.order = -1,
 		.zone = page_zone(pfn_to_page(start)),
-		.mode = MIGRATE_SYNC,
+		.mode = gfp_mask & __GFP_NORETRY ? MIGRATE_ASYNC : MIGRATE_SYNC,
 		.ignore_skip_hint = true,
 		.no_set_skip_hint = true,
 		.gfp_mask = current_gfp_context(gfp_mask),
-- 
2.30.0.296.g2bfb1c46d8-goog