[RFC][PATCH 2/4] memcg: fix error path of mem_cgroup_move_parent

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

There is a bug in error path of mem_cgroup_move_parent.

Extra refcnt got from try_charge should be dropped, and usages incremented
by try_charge should be decremented in both error paths:

    A: failure at get_page_unless_zero
    B: failure at isolate_lru_page

This bug makes this parent directory unremovable.

In case of A, rmdir doesn't return, because res.usage doesn't go
down to 0 at mem_cgroup_force_empty even after all the pc in
lru are removed.
In case of B, rmdir fails and returns -EBUSY, because it has
extra ref counts even after res.usage goes down to 0.


Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
---
 mm/memcontrol.c |   23 +++++++++++++++--------
 1 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 62e69d8..288e22c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -983,14 +983,15 @@ static int mem_cgroup_move_account(struct page_cgroup *pc,
 	if (pc->mem_cgroup != from)
 		goto out;
 
-	css_put(&from->css);
 	res_counter_uncharge(&from->res, PAGE_SIZE);
 	mem_cgroup_charge_statistics(from, pc, false);
 	if (do_swap_account)
 		res_counter_uncharge(&from->memsw, PAGE_SIZE);
+	css_put(&from->css);
+
+	css_get(&to->css);
 	pc->mem_cgroup = to;
 	mem_cgroup_charge_statistics(to, pc, true);
-	css_get(&to->css);
 	ret = 0;
 out:
 	unlock_page_cgroup(pc);
@@ -1023,8 +1024,10 @@ static int mem_cgroup_move_parent(struct page_cgroup *pc,
 	if (ret || !parent)
 		return ret;
 
-	if (!get_page_unless_zero(page))
-		return -EBUSY;
+	if (!get_page_unless_zero(page)) {
+		ret = -EBUSY;
+		goto uncharge;
+	}
 
 	ret = isolate_lru_page(page);
 
@@ -1033,19 +1036,23 @@ static int mem_cgroup_move_parent(struct page_cgroup *pc,
 
 	ret = mem_cgroup_move_account(pc, child, parent);
 
-	/* drop extra refcnt by try_charge() (move_account increment one) */
-	css_put(&parent->css);
 	putback_lru_page(page);
 	if (!ret) {
 		put_page(page);
+		/* drop extra refcnt by try_charge() */
+		css_put(&parent->css);
 		return 0;
 	}
-	/* uncharge if move fails */
+
 cancel:
+	put_page(page);
+uncharge:
+	/* drop extra refcnt by try_charge() */
+	css_put(&parent->css);
+	/* uncharge if move fails */
 	res_counter_uncharge(&parent->res, PAGE_SIZE);
 	if (do_swap_account)
 		res_counter_uncharge(&parent->memsw, PAGE_SIZE);
-	put_page(page);
 	return ret;
 }
 

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