linux-kernel - [PATCH 1/4] slab: do ClearSlabPfmemalloc() for all pages of slab

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Date:	Tue,  4 Sep 2012 18:24:36 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linux-MM <linux-mm@...ck.org>,
	Linux-Netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Chuck Lever <chuck.lever@...cle.com>,
	Joonsoo Kim <js1304@...il.com>, Pekka@...e.de,
	"Enberg <penberg"@kernel.org, David Rientjes <rientjes@...gle.com>,
	Mel Gorman <mgorman@...e.de>
Subject: [PATCH 1/4] slab: do ClearSlabPfmemalloc() for all pages of slab

Right now, we call ClearSlabPfmemalloc() for first page of slab when we
clear SlabPfmemalloc flag. This is fine for most swap-over-network use
cases as it is expected that order-0 pages are in use. Unfortunately it
is possible that that __ac_put_obj() checks SlabPfmemalloc on a tail page
and while this is harmless, it is sloppy. This patch ensures that the head
page is always used.

This problem was originally identified by Joonsoo Kim.

[js1304@...il.com: Original implementation and problem identification]
Signed-off-by: Mel Gorman <mgorman@...e.de>
---
 mm/slab.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index 811af03..d34a903 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1000,7 +1000,7 @@ static void *__ac_get_obj(struct kmem_cache *cachep, struct array_cache *ac,
 		l3 = cachep->nodelists[numa_mem_id()];
 		if (!list_empty(&l3->slabs_free) && force_refill) {
 			struct slab *slabp = virt_to_slab(objp);
-			ClearPageSlabPfmemalloc(virt_to_page(slabp->s_mem));
+			ClearPageSlabPfmemalloc(virt_to_head_page(slabp->s_mem));
 			clear_obj_pfmemalloc(&objp);
 			recheck_pfmemalloc_active(cachep, ac);
 			return objp;
@@ -1032,7 +1032,7 @@ static void *__ac_put_obj(struct kmem_cache *cachep, struct array_cache *ac,
 {
 	if (unlikely(pfmemalloc_active)) {
 		/* Some pfmemalloc slabs exist, check if this is one */
-		struct page *page = virt_to_page(objp);
+		struct page *page = virt_to_head_page(objp);
 		if (PageSlabPfmemalloc(page))
 			set_obj_pfmemalloc(&objp);
 	}
-- 
1.7.9.2

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