mm, page_alloc: fix potential false positive in __zone_watermark_ok

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.

Link: http://lkml.kernel.org/r/20171102125001.23708-1-vbabka@suse.cz
Fixes: 97a16fc82a7c ("mm, page_alloc: only enforce watermarks for order-0 allocations")
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 67f523c4711a2e8d0de4e0599c77cfabdda82efd..04bf1ad50144477c1cb30d9ee2e000fdc954f825 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3041,9 +3041,6 @@ 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;
@@ -3055,6 +3052,9 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
                        return true;
                }
 #endif
+               if (alloc_harder &&
+                       !list_empty(&area->free_list[MIGRATE_HIGHATOMIC]))
+                       return true;
        }
        return false;
 }