From 6920a1bd037374a632d585de127b6f945199dcb8 Mon Sep 17 00:00:00 2001 From: Greg Thelen Date: Fri, 1 Nov 2013 12:16:59 -0700 Subject: [PATCH] memcg: remove incorrect underflow check When a memcg is deleted mem_cgroup_reparent_charges() moves charged memory to the parent memcg. As of v3.11-9444-g3ea67d0 "memcg: add per cgroup writeback pages accounting" there's bad pointer read. The goal was to check for counter underflow. The counter is a per cpu counter and there are two problems with the code: (1) per cpu access function isn't used, instead a naked pointer is used which easily causes oops. (2) the check doesn't sum all cpus Test: $ cd /sys/fs/cgroup/memory $ mkdir x $ echo 3 > /proc/sys/vm/drop_caches $ (echo $BASHPID >> x/tasks && exec cat) & [1] 7154 $ grep ^mapped x/memory.stat mapped_file 53248 $ echo 7154 > tasks $ rmdir x The fix is to remove the check. It's currently dangerous and isn't worth fixing it to use something expensive, such as percpu_counter_sum(), for each reparented page. __this_cpu_read() isn't enough to fix this because there's no guarantees of the current cpus count. The only guarantees is that the sum of all per-cpu counter is >= nr_pages. Fixes: 3ea67d06e467 ("memcg: add per cgroup writeback pages accounting") Reported-and-tested-by: Flavio Leitner Signed-off-by: Greg Thelen Reviewed-by: Sha Zhengju Acked-by: Johannes Weiner Signed-off-by: Hugh Dickins Signed-off-by: Linus Torvalds --- mm/memcontrol.c | 1 - 1 file changed, 1 deletion(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index e63278222be5..13b9d0f221b8 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -3782,7 +3782,6 @@ void mem_cgroup_move_account_page_stat(struct mem_cgroup *from, { /* Update stat data for mem_cgroup */ preempt_disable(); - WARN_ON_ONCE(from->stat->count[idx] < nr_pages); __this_cpu_sub(from->stat->count[idx], nr_pages); __this_cpu_add(to->stat->count[idx], nr_pages); preempt_enable(); -- 2.34.1