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Date:	Mon, 16 Jun 2014 15:54:29 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Tejun Heo <tj@...nel.org>,
	Vladimir Davydov <vdavydov@...allels.com>,
	cgroups@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [patch 09/12] mm: memcontrol: remove ordering between pc->mem_cgroup and PageCgroupUsed

There is a write barrier between setting pc->mem_cgroup and
PageCgroupUsed, which was added to allow LRU operations to lookup the
memcg LRU list of a page without acquiring the page_cgroup lock.

But ever since 38c5d72f3ebe ("memcg: simplify LRU handling by new
rule"), pages are ensured to be off-LRU while charging, so nobody else
is changing LRU state while pc->mem_cgroup is being written, and there
are no read barriers anymore.

Remove the unnecessary write barrier.

Signed-off-by: Johannes Weiner <hannes@...xchg.org>
Acked-by: Michal Hocko <mhocko@...e.cz>
---
 mm/memcontrol.c | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3726f6774860..1cde6e2b33d9 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2801,14 +2801,6 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,
 	}
 
 	pc->mem_cgroup = memcg;
-	/*
-	 * We access a page_cgroup asynchronously without lock_page_cgroup().
-	 * Especially when a page_cgroup is taken from a page, pc->mem_cgroup
-	 * is accessed after testing USED bit. To make pc->mem_cgroup visible
-	 * before USED bit, we need memory barrier here.
-	 * See mem_cgroup_add_lru_list(), etc.
-	 */
-	smp_wmb();
 	SetPageCgroupUsed(pc);
 
 	if (lrucare) {
@@ -3490,7 +3482,6 @@ void mem_cgroup_split_huge_fixup(struct page *head)
 	for (i = 1; i < HPAGE_PMD_NR; i++) {
 		pc = head_pc + i;
 		pc->mem_cgroup = memcg;
-		smp_wmb();/* see __commit_charge() */
 		pc->flags = head_pc->flags & ~PCGF_NOCOPY_AT_SPLIT;
 	}
 	__this_cpu_sub(memcg->stat->count[MEM_CGROUP_STAT_RSS_HUGE],
-- 
2.0.0

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