[PATCH 3/3] Do not unconditionally treat zones that fail zone_reclaim() as full

[Mel Gorman]

On NUMA machines, the administrator can configure zone_reclaim_mode that
is a more targetted form of direct reclaim. On machines with large NUMA
distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
unmapped pages will be reclaimed if the zone watermarks are not being
met. The problem is that zone_reclaim() failing at all means the zone
gets marked full.

This can cause situations where a zone is usable, but is being skipped
because it has been considered full. Take a situation where a large tmpfs
mount is occuping a large percentage of memory overall. The pages do not
get cleaned or reclaimed by zone_reclaim(), but the zone gets marked full
and the zonelist cache considers them not worth trying in the future.

This patch makes zone_reclaim() return more fine-grained information about
what occured when zone_reclaim() failued. The zone only gets marked full if
it really is unreclaimable. If it's a case that the scan did not occur or
if enough pages were not reclaimed with the limited reclaim_mode, then the
zone is simply skipped.

There is a side-effect to this patch. Currently, if zone_reclaim()
successfully reclaimed SWAP_CLUSTER_MAX, an allocation attempt would
go ahead. With this patch applied, zone watermarks are rechecked after
zone_reclaim() does some work.

Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
---
 mm/internal.h   |    4 ++++
 mm/page_alloc.c |   26 ++++++++++++++++++++++----
 mm/vmscan.c     |   10 +++++-----
 3 files changed, 31 insertions(+), 9 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 987bb03..090c267 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -284,4 +284,8 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 		     unsigned long start, int len, int flags,
 		     struct page **pages, struct vm_area_struct **vmas);
 
+#define ZONE_RECLAIM_NOSCAN	-2
+#define ZONE_RECLAIM_FULL	-1
+#define ZONE_RECLAIM_SOME	0
+#define ZONE_RECLAIM_SUCCESS	1
 #endif
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fe753ec..ce2f684 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1420,20 +1420,38 @@ zonelist_scan:
 
 		if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
 			unsigned long mark;
+			int ret;
 			if (alloc_flags & ALLOC_WMARK_MIN)
 				mark = zone->pages_min;
 			else if (alloc_flags & ALLOC_WMARK_LOW)
 				mark = zone->pages_low;
 			else
 				mark = zone->pages_high;
-			if (!zone_watermark_ok(zone, order, mark,
-				    classzone_idx, alloc_flags)) {
-				if (!zone_reclaim_mode ||
-				    !zone_reclaim(zone, gfp_mask, order))
+			if (zone_watermark_ok(zone, order, mark,
+				    classzone_idx, alloc_flags))
+				goto try_this_zone;
+
+			if (zone_reclaim_mode == 0)
+				goto this_zone_full;
+
+			ret = zone_reclaim(zone, gfp_mask, order);
+			switch (ret) {
+				case ZONE_RECLAIM_NOSCAN:
+					/* did not scan */
+					goto try_next_zone;
+				case ZONE_RECLAIM_FULL:
+					/* scanned but unreclaimable */
 					goto this_zone_full;
+				default:
+					/* did we reclaim enough */
+					if (!zone_watermark_ok(zone, order,
+							mark, classzone_idx,
+							alloc_flags))
+						goto try_next_zone;
 			}
 		}
 
+try_this_zone:
 		page = buffered_rmqueue(preferred_zone, zone, order, gfp_mask);
 		if (page)
 			break;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index ffe2f32..84cdae2 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2409,7 +2409,7 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
 	if (pagecache_reclaimable <= zone->min_unmapped_pages
 	    && zone_page_state(zone, NR_SLAB_RECLAIMABLE)
 			<= zone->min_slab_pages)
-		return 0;
+		return ZONE_RECLAIM_NOSCAN;
 
 	/* Do not attempt a scan if scanning failed recently */
 	if (time_before(jiffies,
@@ -2417,13 +2417,13 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
 		return 0;
 
 	if (zone_is_all_unreclaimable(zone))
-		return 0;
+		return ZONE_RECLAIM_FULL;
 
 	/*
 	 * Do not scan if the allocation should not be delayed.
 	 */
 	if (!(gfp_mask & __GFP_WAIT) || (current->flags & PF_MEMALLOC))
-			return 0;
+			return ZONE_RECLAIM_NOSCAN;
 
 	/*
 	 * Only run zone reclaim on the local zone or on zones that do not
@@ -2433,10 +2433,10 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
 	 */
 	node_id = zone_to_nid(zone);
 	if (node_state(node_id, N_CPU) && node_id != numa_node_id())
-		return 0;
+		return ZONE_RECLAIM_NOSCAN;
 
 	if (zone_test_and_set_flag(zone, ZONE_RECLAIM_LOCKED))
-		return 0;
+		return ZONE_RECLAIM_NOSCAN;
 	ret = __zone_reclaim(zone, gfp_mask, order);
 	zone_clear_flag(zone, ZONE_RECLAIM_LOCKED);
 
-- 
1.5.6.5

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