mm: memcontrol: catch root bypass in move precharge

When mem_cgroup_try_charge() returns -EINTR, it bypassed the charge to the
root memcg.  But move precharging does not catch this and treats this case
as if no charge had happened, thus leaking a charge against root.  Because
of an old optimization, the root memcg's res_counter is not actually
charged right now, but it's still an imbalance and subsequent patches will
charge the root memcg again.

Catch those bypasses to the root memcg and properly cancel them before
giving up the move.

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 3c3ab9afc4a16eb592de1d47c4d0d160a49d89cd..d1e450f605bc1d43bad690d02551ddfd807e1bc5 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6426,6 +6426,10 @@ static int mem_cgroup_do_precharge(unsigned long count)
                mc.precharge += count;
                return ret;
        }
+       if (ret == -EINTR) {
+               __mem_cgroup_cancel_charge(root_mem_cgroup, count);
+               return ret;
+       }
 
        /* Try charges one by one with reclaim */
        while (count--) {
@@ -6434,8 +6438,11 @@ static int mem_cgroup_do_precharge(unsigned long count)
                /*
                 * In case of failure, any residual charges against
                 * mc.to will be dropped by mem_cgroup_clear_mc()
-                * later on.
+                * later on.  However, cancel any charges that are
+                * bypassed to root right away or they'll be lost.
                 */
+               if (ret == -EINTR)
+                       __mem_cgroup_cancel_charge(root_mem_cgroup, 1);
                if (ret)
                        return ret;
                mc.precharge++;