lkml.org 
[lkml]   [2010]   [Nov]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    From
    SubjectRe: [BUGFIX][PATCH] fix wrong VM_BUG_ON() in try_charge()'s mm->owner check
    On Wed, 3 Nov 2010, Hiroyuki Kamezawa wrote:
    > I'm sorry for attached file, I have to use unusual mailer this time.
    > This is a fix for wrong VM_BUG_ON() for mm/memcontol.c

    Thanks, Kame, that's good: I've inlined it below with Balbir's Review,
    my Ack, and a Cc: stable@kernel.org.

    Hugh


    [PATCH] 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: Hugh Dickins <hughd@google.com>
    Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Reviewed-by: Balbir Singh <balbir@linux.vnet.ibm.com>
    Acked-by: Hugh Dickins <hughd@google.com>
    Cc: stable@kernel.org
    ---
    mm/memcontrol.c | 19 +++++++++----------
    1 file changed, 9 insertions(+), 10 deletions(-)

    Index: linux-2.6.36/mm/memcontrol.c
    ===================================================================
    --- linux-2.6.36.orig/mm/memcontrol.c
    +++ linux-2.6.36/mm/memcontrol.c
    @@ -1729,19 +1729,18 @@ again:

    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;
    }

    \
     
     \ /
      Last update: 2010-11-04 18:35    [W:0.040 / U:1.176 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site