linux-kernel - [patch 10/11] mm: memcg: only check swap cache pages for repeated charging

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Message-Id: <1341449103-1986-11-git-send-email-hannes@cmpxchg.org>
Date:	Thu,  5 Jul 2012 02:45:02 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [patch 10/11] mm: memcg: only check swap cache pages for repeated charging

Only anon and shmem pages in the swap cache are attempted to be
charged multiple times, from every swap pte fault or from
shmem_unuse().  No other pages require checking PageCgroupUsed().

Charging pages in the swap cache is also serialized by the page lock,
and since both the try_charge and commit_charge are called under the
same page lock section, the PageCgroupUsed() check might as well
happen before the counter charging, let alone reclaim.

Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
 mm/memcontrol.c |   17 ++++++++++++-----
 1 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a8bf86a..d3701cd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2471,11 +2471,7 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,
 	bool anon;
 
 	lock_page_cgroup(pc);
-	if (unlikely(PageCgroupUsed(pc))) {
-		unlock_page_cgroup(pc);
-		__mem_cgroup_cancel_charge(memcg, nr_pages);
-		return;
-	}
+	VM_BUG_ON(PageCgroupUsed(pc));
 	/*
 	 * we don't need page_cgroup_lock about tail pages, becase they are not
 	 * accessed by any other context at this point.
@@ -2740,8 +2736,19 @@ static int __mem_cgroup_try_charge_swapin(struct mm_struct *mm,
 					  struct mem_cgroup **memcgp)
 {
 	struct mem_cgroup *memcg;
+	struct page_cgroup *pc;
 	int ret;
 
+	pc = lookup_page_cgroup(page);
+	/*
+	 * Every swap fault against a single page tries to charge the
+	 * page, bail as early as possible.  shmem_unuse() encounters
+	 * already charged pages, too.  The USED bit is protected by
+	 * the page lock, which serializes swap cache removal, which
+	 * in turn serializes uncharging.
+	 */
+	if (PageCgroupUsed(pc))
+		return 0;
 	if (!do_swap_account)
 		goto charge_cur_mm;
 	/*
-- 
1.7.7.6

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