[patch 09/12] mm: memcontrol: remove ordering between pc->mem_cgroup and PageCgroupUsed

Johannes Weiner
2014-06-16T19:54:29Z

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-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
Acked-by: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>
---
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