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

From: Johannes Weiner
Date: Wed Jul 04 2012 - 20:47:13 EST

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@xxxxxxxxxxx>
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;

- 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;

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