On Mon, 14 Mar 2011, Oleg Nesterov wrote:
> oom_kill_process() simply sets TIF_MEMDIE and returns if PF_EXITING.
> This is very wrong by many reasons. In particular, this thread can
> be the dead group leader. Check p->mm != NULL.
>
This is true only for the oom_kill_allocating_task sysctl where it is
required in all cases to kill current; current won't be triggering the oom
killer if it's dead.
oom_kill_process() is called with the thread selected by
select_bad_process() and that function will not return any thread if any
eligible task is found to be PF_EXITING and is not current, or any
eligible task is found to have TIF_MEMDIE.
In other words, for this conditional to be true in oom_kill_process(),
then p must be current and so it cannot be the dead group leader as
specified in your changelog unless PF_EXITING gets set between
select_bad_process() and the oom_kill_process() call: we don't care about
that since it's in the exit path and we therefore want to give it access
to memory reserves to quickly exit anyway and the check for PF_EXITING in
select_bad_process() prevents any infinite loop of that task getting
constantly reselected if it's dead.
> Note: this is _not_ enough. Just a minimal fix.
>
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> ---
>
> mm/oom_kill.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> --- 38/mm/oom_kill.c~1_kill_fix_pf_exiting 2011-03-14 17:53:05.000000000 +0100
> +++ 38/mm/oom_kill.c 2011-03-14 18:51:49.000000000 +0100
> @@ -470,7 +470,7 @@ static int oom_kill_process(struct task_
> * If the task is already exiting, don't alarm the sysadmin or kill
> * its children or threads, just set TIF_MEMDIE so it can die quickly
> */
> - if (p->flags & PF_EXITING) {
> + if (p->flags & PF_EXITING && p->mm) {
> set_tsk_thread_flag(p, TIF_MEMDIE);
> boost_dying_task_prio(p, mem);
> return 0;
>
>
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