mm: memcg: only check swap cache pages for repeated charging

hnaz · torvalds · commit 90deb78839fa · 2012-07-31T18:42:49.000-07:00
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 &lt;hannes@cmpxchg.org&gt;
Acked-by: KAMEZAWA Hiroyuki &lt;kamezawa.hiroyu@jp.fujitsu.com&gt;
Acked-by: Michal Hocko &lt;mhocko@suse.cz&gt;
Cc: David Rientjes &lt;rientjes@google.com&gt;
Cc: Hugh Dickins &lt;hughd@google.com&gt;
Cc: Johannes Weiner &lt;hannes@cmpxchg.org&gt;
Cc: Wanpeng Li &lt;liwp.linux@gmail.com&gt;
Cc: Mel Gorman &lt;mel@csn.ul.ie&gt;
Signed-off-by: Andrew Morton &lt;akpm@linux-foundation.org&gt;
Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
@@ -2538,11 +2538,7 @@ 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.
@@ -2807,8 +2803,19 @@ 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;
 	/*
