Patch "mm, page_alloc: fix potential false positive in __zone_watermark_ok" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

mm, page_alloc: fix potential false positive in __zone_watermark_ok

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-page_alloc-fix-potential-false-positive-in-__zone_watermark_ok.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Vlastimil Babka <[email protected]>
Date: Wed, 15 Nov 2017 17:38:30 -0800
Subject: mm, page_alloc: fix potential false positive in __zone_watermark_ok

From: Vlastimil Babka <[email protected]>

commit b050e3769c6b4013bb937e879fc43bf1847ee819 upstream.

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.

---
 mm/page_alloc.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

@@ -2468,9 +2468,6 @@ static bool __zone_watermark_ok(struct z
 		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;

@@ -2482,6 +2479,9 @@ static bool __zone_watermark_ok(struct z
 			return true;
 		}
 #endif
+		if (alloc_harder &&
+			!list_empty(&area->free_list[MIGRATE_HIGHATOMIC]))
+			return true;
 	}
 	return false;
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/fs-select-add-vmalloc-fallback-for-select-2.patch queue-4.4/mm-mmap.c-do-not-blow-on-prot_none-map_fixed-holes-in-the-stack.patch queue-4.4/cma-fix-calculation-of-aligned-offset.patch queue-4.4/mm-page_alloc-fix-potential-false-positive-in-__zone_watermark_ok.patch