[3.16.y-ckt extended stable] Patch "mm: page_alloc: fix zone allocation fairness on UP" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    mm: page_alloc: fix zone allocation fairness on UP

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt1.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 3ad339fb32ddc9b285264809286c8587a3bad3e0 Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes at cmpxchg.org>
Date: Thu, 2 Oct 2014 16:21:10 -0700
Subject: mm: page_alloc: fix zone allocation fairness on UP

commit abe5f972912d086c080be4bde67750630b6fb38b upstream.

The zone allocation batches can easily underflow due to higher-order
allocations or spills to remote nodes.  On SMP that's fine, because
underflows are expected from concurrency and dealt with by returning 0.
But on UP, zone_page_state will just return a wrapped unsigned long,
which will get past the <= 0 check and then consider the zone eligible
until its watermarks are hit.

Commit 3a025760fc15 ("mm: page_alloc: spill to remote nodes before
waking kswapd") already made the counter-resetting use
atomic_long_read() to accomodate underflows from remote spills, but it
didn't go all the way with it.

Make it clear that these batches are expected to go negative regardless
of concurrency, and use atomic_long_read() everywhere.

Fixes: 81c0a2bb515f ("mm: page_alloc: fair zone allocator policy")
Reported-by: Vlastimil Babka <vbabka at suse.cz>
Reported-by: Leon Romanovsky <leon at leon.nu>
Signed-off-by: Johannes Weiner <hannes at cmpxchg.org>
Acked-by: Mel Gorman <mgorman at suse.de>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
[ luis: backported to 3.16 by Johannes Weiner ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 mm/page_alloc.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ef44ad736ca1..1cd18c7d2d20 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1965,7 +1965,7 @@ zonelist_scan:
 		if (alloc_flags & ALLOC_FAIR) {
 			if (!zone_local(preferred_zone, zone))
 				continue;
-			if (zone_page_state(zone, NR_ALLOC_BATCH) <= 0)
+			if (atomic_long_read(&zone->vm_stat[NR_ALLOC_BATCH]) <= 0)
 				continue;
 		}
 		/*
@@ -5694,9 +5694,8 @@ static void __setup_per_zone_wmarks(void)
 		zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + (tmp >> 1);

 		__mod_zone_page_state(zone, NR_ALLOC_BATCH,
-				      high_wmark_pages(zone) -
-				      low_wmark_pages(zone) -
-				      zone_page_state(zone, NR_ALLOC_BATCH));
+			high_wmark_pages(zone) - low_wmark_pages(zone) -
+			atomic_long_read(&zone->vm_stat[NR_ALLOC_BATCH]));

 		setup_zone_migrate_reserve(zone);
 		spin_unlock_irqrestore(&zone->lock, flags);
--
2.1.0