Commit 9a2385ee authored Aug 06, 2014 by Johannes Weiner Committed by Linus Torvalds Aug 06, 2014

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 commit 38c5d72f ("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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 05b84301

mm/memcontrol.c

+0 −9

Original line number	Diff line number	Diff line
		@@ -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) {
		@@ -3483,7 +3475,6 @@ 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],