Patch "mm, page_alloc: fix potential false positive in __zone_watermark_ok" has been added to the 4.9-stable tree

[<gregkh@linuxfoundation.org>]

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 4.9-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-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From b050e3769c6b4013bb937e879fc43bf1847ee819 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@suse.cz>
Date: Wed, 15 Nov 2017 17:38:30 -0800
Subject: mm, page_alloc: fix potential false positive in __zone_watermark_ok

From: Vlastimil Babka <vbabka@suse.cz>

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.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2821,9 +2821,6 @@ bool __zone_watermark_ok(struct zone *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;
@@ -2835,6 +2832,9 @@ bool __zone_watermark_ok(struct zone *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 vbabka@suse.cz are

queue-4.9/mm-mmap.c-do-not-blow-on-prot_none-map_fixed-holes-in-the-stack.patch
queue-4.9/cma-fix-calculation-of-aligned-offset.patch
queue-4.9/mm-page_alloc-fix-potential-false-positive-in-__zone_watermark_ok.patch