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

[Johannes Weiner <hannes@cmpxchg.org>]

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@cmpxchg.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