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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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