mm: memcontrol: remove ordering between pc->mem_cgroup and PageCgroupUsed

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@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index dae8c2fdab8358cfe26718fcebe6011493552771..0b8df1f2b0a363c782d2cf3895ae3b0e4875853f 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2795,14 +2795,6 @@ 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) {
@@ -3534,7 +3526,6 @@ 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],