[PATCH 8/9] mm: zone_reclaim: after a successful zone_reclaim check the min watermark

[Andrea Arcangeli <aarcange@redhat.com>]

If we're in the fast path and we succeeded zone_reclaim(), it means we
freed enough memory and we can use the min watermark to have some
margin against concurrent allocations from other CPUs or interrupts.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 mm/page_alloc.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b32ecde..879a3fd 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1990,8 +1990,26 @@ zonelist_scan:
 			case ZONE_RECLAIM_FULL:
 				/* scanned but unreclaimable */
 				continue;
+			case ZONE_RECLAIM_SUCCESS:
+				/*
+				 * If we successfully reclaimed
+				 * enough, allow allocations up to the
+				 * min watermark (instead of stopping
+				 * at "mark"). This provides some more
+				 * margin against parallel
+				 * allocations. Using the min
+				 * watermark doesn't alter when we
+				 * wakeup kswapd. It also doesn't
+				 * alter the synchronous direct
+				 * reclaim behavior of zone_reclaim()
+				 * that will still be invoked at the
+				 * next pass if we're still below the
+				 * low watermark (even if kswapd isn't
+				 * woken).
+				 */
+				mark = min_wmark_pages(zone);
+				/* Fall through */
 			default:
-				/* did we reclaim enough */
 				if (zone_watermark_ok(zone, order, mark,
 						classzone_idx, alloc_flags))
 					goto try_this_zone;

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