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Commit ebb76ce1 authored by KAMEZAWA Hiroyuki's avatar KAMEZAWA Hiroyuki Committed by Linus Torvalds
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memcg: fix wrong VM_BUG_ON() in try_charge()'s mm->owner check 



At __mem_cgroup_try_charge(), VM_BUG_ON(!mm->owner) is checked.
But as commented in mem_cgroup_from_task(), mm->owner can be NULL
in some racy case. This check of VM_BUG_ON() is bad.

A possible story to hit this is at swapoff()->try_to_unuse(). It passes
mm_struct to mem_cgroup_try_charge_swapin() while mm->owner is NULL. If we
can't get proper mem_cgroup from swap_cgroup information, mm->owner is used
as charge target and we see NULL.

Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reported-by: default avatarHugh Dickins <hughd@google.com>
Reported-by: default avatarThomas Meyer <thomas@m3y3r.de>
Signed-off-by: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: default avatarBalbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: default avatarHugh Dickins <hughd@google.com>
Cc: stable@kernel.org
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent b83be6f2

mm/memcontrol.c

+9 −10
Original line number Diff line number Diff line
@@ -1925,19 +1925,18 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm, 


		rcu_read_lock();

		p = rcu_dereference(mm->owner);

		VM_BUG_ON(!p);

		/*

		 * because we don't have task_lock(), "p" can exit while

		 * we're here. In that case, "mem" can point to root

		 * cgroup but never be NULL. (and task_struct itself is freed

		 * by RCU, cgroup itself is RCU safe.) Then, we have small

		 * risk here to get wrong cgroup. But such kind of mis-account

		 * by race always happens because we don't have cgroup_mutex().

		 * It's overkill and we allow that small race, here.

		 * Because we don't have task_lock(), "p" can exit.

		 * In that case, "mem" can point to root or p can be NULL with

		 * race with swapoff. Then, we have small risk of mis-accouning.

		 * But such kind of mis-account by race always happens because

		 * we don't have cgroup_mutex(). It's overkill and we allo that

		 * small race, here.

		 * (*) swapoff at el will charge against mm-struct not against

		 * task-struct. So, mm->owner can be NULL.

		 */

		mem = mem_cgroup_from_task(p);

		VM_BUG_ON(!mem);

		if (mem_cgroup_is_root(mem)) {

		if (!mem || mem_cgroup_is_root(mem)) {

			rcu_read_unlock();

			goto done;

		}