[PATCH] mm, page_alloc: fix potential false positive in __zone_watermark_ok

[Vlastimil Babka]

Since commit 97a16fc82a7c ("mm, page_alloc: only enforce watermarks for order-0
allocations"), __zone_watermark_ok() check for high-order allocations will
shortcut per-migratetype free list checks for ALLOC_HARDER allocations, and
return true as long as there's free page of any migratetype. The intention is
that ALLOC_HARDER can allocate from MIGRATE_HIGHATOMIC free lists, while normal
allocations can't.

However, as a side effect, the watermark check will then also return true when
there are pages only on the MIGRATE_ISOLATE list, or (prior to CMA conversion
to ZONE_MOVABLE) on the MIGRATE_CMA list. Since the allocation cannot actually
obtain isolated pages, and might not be able to obtain CMA pages, this can
result in a false positive.

The condition should be rare and perhaps the outcome is not a fatal one. Still,
it's better if the watermark check is correct. There also shouldn't be a
performance tradeoff here.

Fixes: 97a16fc82a7c ("mm, page_alloc: only enforce watermarks for order-0 allocations")
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/page_alloc.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 79cdac1fee42..f43039945148 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3032,9 +3032,6 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
 		if (!area->nr_free)
 			continue;
 
-		if (alloc_harder)
-			return true;
-
 		for (mt = 0; mt < MIGRATE_PCPTYPES; mt++) {
 			if (!list_empty(&area->free_list[mt]))
 				return true;
@@ -3044,6 +3041,9 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
 		if (!list_empty(&area->free_list[MIGRATE_CMA]))
 			return true;
 #endif
+		if (alloc_harder &&
+			!list_empty(&area->free_list[MIGRATE_HIGHATOMIC]))
+			return true;
 	}
 	return false;
 }
-- 
2.14.3