On Sat 16-07-16 14:30:04, Tetsuo Handa wrote:
> Patch "mm, oom: hide mm which is shared with kthread or global init" tried
> to guarantee a forward progress for the OOM killer even when the selected
> victim is sharing memory with a kernel thread or global init, but a race
> scenario still remains because it did not add a call to exit_oom_victim()
> in oom_kill_process() in order to avoid a problem which is already worked
> around by commit 74070542099c66d8 ("oom, suspend: fix oom_reaper vs.
> oom_killer_disable race").
>
> The race scenario is that a !can_oom_reap TIF_MEMDIE thread becomes
> the only user of that mm (i.e. mm->mm_users drops to 1) and is later
> blocked for unbounded period at __mmput() from mmput() from
> exit_mm() from do_exit() by hitting e.g.
>
> (1) First round of OOM killer invocation starts.
> (2) select_bad_process() chooses P1 as an OOM victim because
> oom_scan_process_thread() does not find existing victims.
> (3) oom_kill_process() sets TIF_MEMDIE on P1, but does not put P1 under
> the OOM reaper's supervision due to (p->flags & PF_KTHREAD) being
> true, and instead sets MMF_OOM_REAPED on the P1's mm.
> (4) First round of OOM killer invocation finishes.
> (5) P1 is unable to arrive at do_exit() due to being blocked at
> unkillable event waiting for somebody else's memory allocation.
> (6) Second round of OOM killer invocation starts.
> (7) select_bad_process() chooses P2 as an OOM victim because
> oom_scan_process_thread() finds P1's mm with MMF_OOM_REAPED set.
> (8) oom_kill_process() sets TIF_MEMDIE on P2 via mark_oom_victim(),
> and puts P2 under the OOM reaper's supervision due to
> (p->flags & PF_KTHREAD) being false.
> (9) Second round of OOM killer invocation finishes.
> (10) The OOM reaper reaps P2's mm, and sets MMF_OOM_REAPED to
> P2's mm, and clears TIF_MEMDIE from P2.
> (11) Regarding P1's mm, (p->flags & PF_KTHREAD) becomes false because
> somebody else's memory allocation succeeds and unuse_mm(P1->mm)
> is called. At this point P1 becomes the only user of P1->mm.
> (12) P1 arrives at do_exit() due to no longer being blocked at
> unkillable event waiting for somebody else's memory allocation.
> (13) P1 reaches P1->mm = NULL line in exit_mm() from do_exit().
> (14) P1 is blocked at __mmput().
> (15) Third round of OOM killer invocation starts.
> (16) select_bad_process() does not choose new OOM victim because
> oom_scan_process_thread() fails to find P1's mm while
> P1->signal->oom_victims > 0.
> (17) Third round of OOM killer invocation finishes.
> (18) OOM livelock happens because nobody will clear TIF_MEMDIE from
> P1 (and decrement P1->signal->oom_victims) while P1 is blocked
> at __mmput().
>
> sequence, but the patch "mm, oom: hide mm which is shared with kthread
> or global init" is failing to return OOM_SCAN_CONTINUE when we hit
> atomic_read(&task->signal->oom_victims) != 0 &&
> find_lock_task_mm(task) == NULL in oom_scan_process_thread().
>
> Long term we are planning to change oom_scan_process_thread() not to
> depend on atomic_read(&task->signal->oom_victims) != 0 &&
> find_lock_task_mm(task) != NULL, and remove exit_oom_victim() from
> oom_kill_process() and oom_reap_task() along with signal->oom_victims
> and commit 74070542099c66d8. But since we did not complete such changes
> in time for 4.8 merge window, let's rely on commit 74070542099c66d8
> for now in order to guarantee a forward progress for the OOM killer.
I really do not think that this unlikely case really has to be handled
now. We are very likely going to move to a different model of oom victim
detection soon. So let's do not add new hacks. exit_oom_victim from
oom_kill_process just looks like sand in eyes.
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> mm/oom_kill.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 7d0a275..041373e 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -922,6 +922,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
> */
> can_oom_reap = false;
> set_bit(MMF_OOM_REAPED, &mm->flags);
> + exit_oom_victim(victim);
> pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n",
> task_pid_nr(victim), victim->comm,
> task_pid_nr(p), p->comm);
> --
> 1.8.3.1
>
--
Michal Hocko
SUSE Labs
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