memcg: remove PCG_MOVE_LOCK flag from page_cgroup

PCG_MOVE_LOCK is used for bit spinlock to avoid race between overwriting
pc->mem_cgroup and page statistics accounting per memcg.  This lock helps
to avoid the race but the race is very rare because moving tasks between
cgroup is not a usual job.  So, it seems using 1bit per page is too
costly.

This patch changes this lock as per-memcg spinlock and removes
PCG_MOVE_LOCK.

If smaller lock is required, we'll be able to add some hashes but I'd like
to start from this.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Greg Thelen <gthelen@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 32 insertions, 10 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index cfd2db08cfe1..8afed2819b8f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -300,6 +300,8 @@ struct mem_cgroup {
          * set > 0 if pages under this cgroup are moving to other cgroup.
          */
         atomic_t        moving_account;
+        /* taken only while moving_account > 0 */
+        spinlock_t      move_lock;
         /*
          * percpu counter.
          */
@@ -1376,6 +1378,24 @@ static bool mem_cgroup_wait_acct_move(struct mem_cgroup *memcg)
         return false;
 }
 
+/*
+ * Take this lock when
+ * - a code tries to modify page's memcg while it's USED.
+ * - a code tries to modify page state accounting in a memcg.
+ * see mem_cgroup_stealed(), too.
+ */
+static void move_lock_mem_cgroup(struct mem_cgroup *memcg,
+                                  unsigned long *flags)
+{
+        spin_lock_irqsave(&memcg->move_lock, *flags);
+}
+
+static void move_unlock_mem_cgroup(struct mem_cgroup *memcg,
+                                unsigned long *flags)
+{
+        spin_unlock_irqrestore(&memcg->move_lock, *flags);
+}
+
 /**
  * mem_cgroup_print_oom_info: Called from OOM with tasklist_lock held in read mode.
  * @memcg: The memory cgroup that went over limit
@@ -1900,7 +1920,7 @@ void mem_cgroup_update_page_stat(struct page *page,
 
         if (mem_cgroup_disabled())
                 return;
-
+again:
         rcu_read_lock();
         memcg = pc->mem_cgroup;
         if (unlikely(!memcg || !PageCgroupUsed(pc)))
@@ -1908,11 +1928,13 @@ void mem_cgroup_update_page_stat(struct page *page,
         /* pc->mem_cgroup is unstable ? */
         if (unlikely(mem_cgroup_stealed(memcg))) {
                 /* take a lock against to access pc->mem_cgroup */
-                move_lock_page_cgroup(pc, &flags);
+                move_lock_mem_cgroup(memcg, &flags);
+                if (memcg != pc->mem_cgroup || !PageCgroupUsed(pc)) {
+                        move_unlock_mem_cgroup(memcg, &flags);
+                        rcu_read_unlock();
+                        goto again;
+                }
                 need_unlock = true;
-                memcg = pc->mem_cgroup;
-                if (!memcg || !PageCgroupUsed(pc))
-                        goto out;
         }
 
         switch (idx) {
@@ -1931,7 +1953,7 @@ void mem_cgroup_update_page_stat(struct page *page,
 
 out:
         if (unlikely(need_unlock))
-                move_unlock_page_cgroup(pc, &flags);
+                move_unlock_mem_cgroup(memcg, &flags);
         rcu_read_unlock();
 }
 
@@ -2500,8 +2522,7 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 
-#define PCGF_NOCOPY_AT_SPLIT ((1 << PCG_LOCK) | (1 << PCG_MOVE_LOCK) |\
-                        (1 << PCG_MIGRATION))
+#define PCGF_NOCOPY_AT_SPLIT ((1 << PCG_LOCK) | (1 << PCG_MIGRATION))
 /*
  * Because tail pages are not marked as "used", set it. We're under
  * zone->lru_lock, 'splitting on pmd' and compound_lock.
@@ -2572,7 +2593,7 @@ static int mem_cgroup_move_account(struct page *page,
         if (!PageCgroupUsed(pc) || pc->mem_cgroup != from)
                 goto unlock;
 
-        move_lock_page_cgroup(pc, &flags);
+        move_lock_mem_cgroup(from, &flags);
 
         if (PageCgroupFileMapped(pc)) {
                 /* Update mapped_file data for mem_cgroup */
@@ -2596,7 +2617,7 @@ static int mem_cgroup_move_account(struct page *page,
          * guaranteed that "to" is never removed. So, we don't check rmdir
          * status here.
          */
-        move_unlock_page_cgroup(pc, &flags);
+        move_unlock_mem_cgroup(from, &flags);
         ret = 0;
 unlock:
         unlock_page_cgroup(pc);
@@ -4971,6 +4992,7 @@ mem_cgroup_create(struct cgroup *cont)
         atomic_set(&memcg->refcnt, 1);
         memcg->move_charge_at_immigrate = 0;
         mutex_init(&memcg->thresholds_lock);
+        spin_lock_init(&memcg->move_lock);
         return &memcg->css;
 free_out:
         __mem_cgroup_free(memcg);