The patch titled
Subject: mm: memcontrol: remove ordering between pc->mem_cgroup and PageCgroupUsed
has been added to the -mm tree. Its filename is
mm-memcontrol-remove-ordering-between-pc-mem_cgroup-and-pagecgroupused.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-memcontrol-remove-ordering-between-pc-mem_cgroup-and-pagecgroupused.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-memcontrol-remove-ordering-between-pc-mem_cgroup-and-pagecgroupused.patch
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------------------------------------------------------
From: Johannes Weiner <hannes@xxxxxxxxxxx>
Subject: 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@xxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/memcontrol.c | 9 ---------
1 file changed, 9 deletions(-)
diff -puN mm/memcontrol.c~mm-memcontrol-remove-ordering-between-pc-mem_cgroup-and-pagecgroupused mm/memcontrol.c
--- a/mm/memcontrol.c~mm-memcontrol-remove-ordering-between-pc-mem_cgroup-and-pagecgroupused
+++ a/mm/memcontrol.c
@@ -2795,14 +2795,6 @@ static void __mem_cgroup_commit_charge(s
}
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
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],
_
Patches currently in -mm which might be from hannes@xxxxxxxxxxx are
vmalloc-use-rcu-list-iterator-to-reduce-vmap_area_lock-contention.patch
memcg-cleanup-memcg_cache_params-refcnt-usage.patch
memcg-destroy-kmem-caches-when-last-slab-is-freed.patch
memcg-mark-caches-that-belong-to-offline-memcgs-as-dead.patch
slub-dont-fail-kmem_cache_shrink-if-slab-placement-optimization-fails.patch
slub-make-slab_free-non-preemptable.patch
memcg-wait-for-kfrees-to-finish-before-destroying-cache.patch
slub-make-dead-memcg-caches-discard-free-slabs-immediately.patch
slab-do-not-keep-free-objects-slabs-on-dead-memcg-caches.patch
mm-memcontrol-fold-mem_cgroup_do_charge.patch
mm-memcontrol-rearrange-charging-fast-path.patch
mm-memcontrol-reclaim-at-least-once-for-__gfp_noretry.patch
mm-huge_memory-use-gfp_transhuge-when-charging-huge-pages.patch
mm-memcontrol-retry-reclaim-for-oom-disabled-and-__gfp_nofail-charges.patch
mm-memcontrol-remove-explicit-oom-parameter-in-charge-path.patch
mm-memcontrol-simplify-move-precharge-function.patch
mm-memcontrol-catch-root-bypass-in-move-precharge.patch
mm-memcontrol-use-root_mem_cgroup-res_counter.patch
mm-memcontrol-remove-ordering-between-pc-mem_cgroup-and-pagecgroupused.patch
mm-memcontrol-do-not-acquire-page_cgroup-lock-for-kmem-pages.patch
mm-memcontrol-rewrite-charge-api.patch
mm-memcontrol-rewrite-uncharge-api.patch
memcg-deprecate-memoryforce_empty-knob.patch
memcg-deprecate-memoryforce_empty-knob-fix.patch
debugging-keep-track-of-page-owners.patch
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