cgroup avoid permanent sleep at rmdir

After commit ec64f51545fffbc4cb968f0cea56341a4b07e85a ("cgroup: fix
frequent -EBUSY at rmdir"), cgroup's rmdir (especially against memcg)
doesn't return -EBUSY by temporary ref counts.  That commit expects all
refs after pre_destroy() is temporary but...it wasn't.  Then, rmdir can
wait permanently.  This patch tries to fix that and change followings.

 - set CGRP_WAIT_ON_RMDIR flag before pre_destroy().
 - clear CGRP_WAIT_ON_RMDIR flag when the subsys finds racy case.
   if there are sleeping ones, wakes them up.
 - rmdir() sleeps only when CGRP_WAIT_ON_RMDIR flag is set.

Tested-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Reported-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Reviewed-by: Paul Menage <menage@google.com>
Acked-by: Balbir Sigh <balbir@linux.vnet.ibm.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 20 insertions, 3 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e717964cb5a0..fd4529d86de5 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1207,6 +1207,12 @@ static int mem_cgroup_move_account(struct page_cgroup *pc,
         ret = 0;
 out:
         unlock_page_cgroup(pc);
+        /*
+         * We charges against "to" which may not have any tasks. Then, "to"
+         * can be under rmdir(). But in current implementation, caller of
+         * this function is just force_empty() and it's garanteed that
+         * "to" is never removed. So, we don't check rmdir status here.
+         */
         return ret;
 }
 
@@ -1428,6 +1434,7 @@ __mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *ptr,
                 return;
         if (!ptr)
                 return;
+        cgroup_exclude_rmdir(&ptr->css);
         pc = lookup_page_cgroup(page);
         mem_cgroup_lru_del_before_commit_swapcache(page);
         __mem_cgroup_commit_charge(ptr, pc, ctype);
@@ -1457,8 +1464,12 @@ __mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *ptr,
                 }
                 rcu_read_unlock();
         }
-        /* add this page(page_cgroup) to the LRU we want. */
-
+        /*
+         * At swapin, we may charge account against cgroup which has no tasks.
+         * So, rmdir()->pre_destroy() can be called while we do this charge.
+         * In that case, we need to call pre_destroy() again. check it here.
+         */
+        cgroup_release_and_wakeup_rmdir(&ptr->css);
 }
 
 void mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *ptr)
@@ -1664,7 +1675,7 @@ void mem_cgroup_end_migration(struct mem_cgroup *mem,
 
         if (!mem)
                 return;
-
+        cgroup_exclude_rmdir(&mem->css);
         /* at migration success, oldpage->mapping is NULL. */
         if (oldpage->mapping) {
                 target = oldpage;
@@ -1704,6 +1715,12 @@ void mem_cgroup_end_migration(struct mem_cgroup *mem,
          */
         if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
                 mem_cgroup_uncharge_page(target);
+        /*
+         * At migration, we may charge account against cgroup which has no tasks
+         * So, rmdir()->pre_destroy() can be called while we do this charge.
+         * In that case, we need to call pre_destroy() again. check it here.
+         */
+        cgroup_release_and_wakeup_rmdir(&mem->css);
 }
 
 /*