mm: memcontrol: shorten the page statistics update slowpath

While moving charges from one memcg to another, page stat updates must
acquire the old memcg's move_lock to prevent double accounting.  That
situation is denoted by an increased memcg->move_accounting.  However, the
charge moving code declares this way too early for now, even before
summing up the RSS and pre-allocating destination charges.

Shorten this slowpath mode by increasing memcg->move_accounting only right
before walking the task's address space with the intention of actually
moving the pages.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 13 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 9d30129b0d4a..9073d07c1149 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5333,8 +5333,6 @@ static void __mem_cgroup_clear_mc(void)
 
 static void mem_cgroup_clear_mc(void)
 {
-        struct mem_cgroup *from = mc.from;
-
         /*
          * we must clear moving_task before waking up waiters at the end of
          * task migration.
@@ -5345,8 +5343,6 @@ static void mem_cgroup_clear_mc(void)
         mc.from = NULL;
         mc.to = NULL;
         spin_unlock(&mc.lock);
-
-        atomic_dec(&from->moving_account);
 }
 
 static int mem_cgroup_can_attach(struct cgroup_subsys_state *css,
@@ -5380,15 +5376,6 @@ static int mem_cgroup_can_attach(struct cgroup_subsys_state *css,
                         VM_BUG_ON(mc.moved_charge);
                         VM_BUG_ON(mc.moved_swap);
 
-                        /*
-                         * Signal mem_cgroup_begin_page_stat() to take
-                         * the memcg's move_lock while we're moving
-                         * its pages to another memcg.  Then wait for
-                         * already started RCU-only updates to finish.
-                         */
-                        atomic_inc(&from->moving_account);
-                        synchronize_rcu();
-
                         spin_lock(&mc.lock);
                         mc.from = from;
                         mc.to = memcg;
@@ -5520,6 +5507,13 @@ static void mem_cgroup_move_charge(struct mm_struct *mm)
         struct vm_area_struct *vma;
 
         lru_add_drain_all();
+        /*
+         * Signal mem_cgroup_begin_page_stat() to take the memcg's
+         * move_lock while we're moving its pages to another memcg.
+         * Then wait for already started RCU-only updates to finish.
+         */
+        atomic_inc(&mc.from->moving_account);
+        synchronize_rcu();
 retry:
         if (unlikely(!down_read_trylock(&mm->mmap_sem))) {
                 /*
@@ -5552,6 +5546,7 @@ retry:
                         break;
         }
         up_read(&mm->mmap_sem);
+        atomic_dec(&mc.from->moving_account);
 }
 
 static void mem_cgroup_move_task(struct cgroup_subsys_state *css,