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    SubjectRe: [patch 05/11 -mmotm] oom: fix possible oom_dump_tasks NULL pointer
    On Sun, May 10, 2009 at 03:07:16PM -0700, David Rientjes wrote:
    > When /proc/sys/vm/oom_dump_tasks is enabled, it is possible to get a NULL
    > pointer for tasks that have detached mm's since task_lock() is not held
    > during the tasklist scan.
    >
    > Acked-by: Nick Piggin <npiggin@suse.de>
    > Signed-off-by: David Rientjes <rientjes@google.com>

    Minor nit. The Acked-by should come after the Signed-off-by.

    The window during which p->mm was a valid point and become an invalid
    one is *extremely* small but sure, it's there.

    Acked-by: Mel Gorman <mel@csn.ul.ie>

    I would have preferred if the core VM patches were in a different set to
    the driver patches. Maybe there is an interdependency later but this patch
    at least looks standalone.

    > ---
    > mm/oom_kill.c | 24 +++++++++++++++---------
    > 1 files changed, 15 insertions(+), 9 deletions(-)
    >
    > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
    > --- a/mm/oom_kill.c
    > +++ b/mm/oom_kill.c
    > @@ -284,22 +284,28 @@ static void dump_tasks(const struct mem_cgroup *mem)
    > printk(KERN_INFO "[ pid ] uid tgid total_vm rss cpu oom_adj "
    > "name\n");
    > do_each_thread(g, p) {
    > - /*
    > - * total_vm and rss sizes do not exist for tasks with a
    > - * detached mm so there's no need to report them.
    > - */
    > - if (!p->mm)
    > - continue;
    > + struct mm_struct *mm;
    > +
    > if (mem && !task_in_mem_cgroup(p, mem))
    > continue;
    > if (!thread_group_leader(p))
    > continue;
    >
    > task_lock(p);
    > + mm = p->mm;
    > + if (!mm) {
    > + /*
    > + * total_vm and rss sizes do not exist for tasks with no
    > + * mm so there's no need to report them; they can't be
    > + * oom killed anyway.
    > + */
    > + task_unlock(p);
    > + continue;
    > + }
    > printk(KERN_INFO "[%5d] %5d %5d %8lu %8lu %3d %3d %s\n",
    > - p->pid, __task_cred(p)->uid, p->tgid,
    > - p->mm->total_vm, get_mm_rss(p->mm), (int)task_cpu(p),
    > - p->oomkilladj, p->comm);
    > + p->pid, __task_cred(p)->uid, p->tgid, mm->total_vm,
    > + get_mm_rss(mm), (int)task_cpu(p), p->oomkilladj,
    > + p->comm);
    > task_unlock(p);
    > } while_each_thread(g, p);
    > }
    >

    --
    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab


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