linux-kernel - [PATCH] mm: page_alloc: Fix setting of ZONE_FAIR

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Date:	Fri, 5 Sep 2014 11:14:51 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>
Subject: [PATCH] mm: page_alloc: Fix setting of ZONE_FAIR_DEPLETED on UP

Commit 4ffeaf35 (mm: page_alloc: reduce cost of the fair zone allocation policy)
broke the fair zone allocation policy on UP with these hunks.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6e5e8f7..fb99081 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1612,6 +1612,9 @@ again:
        }

        __mod_zone_page_state(zone, NR_ALLOC_BATCH, -(1 << order));
+       if (zone_page_state(zone, NR_ALLOC_BATCH) == 0 &&
+           !zone_is_fair_depleted(zone))
+               zone_set_flag(zone, ZONE_FAIR_DEPLETED);

        __count_zone_vm_events(PGALLOC, zone, 1 << order);
        zone_statistics(preferred_zone, zone, gfp_flags);
@@ -1966,8 +1985,10 @@ zonelist_scan:
                if (alloc_flags & ALLOC_FAIR) {
                        if (!zone_local(preferred_zone, zone))
                                break;
-                       if (zone_page_state(zone, NR_ALLOC_BATCH) <= 0)
+                       if (zone_is_fair_depleted(zone)) {
+                               nr_fair_skipped++;
                                continue;
+                       }
                }

The problem is that a <= check was replaced with a ==. On SMP it doesn't
matter because negative values are returned as zero due to per-CPU drift
which is not possible in the UP case. Vlastimil Babka correctly pointed
out that this can be negative due to high-order allocations. This patch
fixes the problem.

Reported-by: Vlastimil Babka <vbabka@...e.cz>
Signed-off-by: Mel Gorman <mgorman@...e.de>
---
 mm/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 18cee0d..cd4c05c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1612,7 +1612,7 @@ again:
 	}
 
 	__mod_zone_page_state(zone, NR_ALLOC_BATCH, -(1 << order));
-	if (zone_page_state(zone, NR_ALLOC_BATCH) == 0 &&
+	if (zone_page_state(zone, NR_ALLOC_BATCH) <= 0 &&
 	    !zone_is_fair_depleted(zone))
 		zone_set_flag(zone, ZONE_FAIR_DEPLETED);
 
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