Andrew,
On Mon 20-06-16 14:43:48, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
>
> The only case where the oom_reaper is not triggered for the oom victim
> is when it shares the memory with a kernel thread (aka use_mm) or with
> the global init. After "mm, oom: skip vforked tasks from being selected"
> the victim cannot be a vforked task of the global init so we are left
> with clone(CLONE_VM) (without CLONE_SIGHAND). use_mm() users are quite
> rare as well.
>
> In order to guarantee a forward progress for the OOM killer make
> sure that this really rare cases will not get into the way and hide
> the mm from the oom killer by setting MMF_OOM_REAPED flag for it.
> oom_scan_process_thread will ignore any TIF_MEMDIE task if it has
> MMF_OOM_REAPED flag set to catch these oom victims.
>
> After this patch we should guarantee a forward progress for the OOM
> killer even when the selected victim is sharing memory with a kernel
> thread or global init.
Could you replace the last two paragraphs with the following. Tetsuo
didn't like the guarantee mentioned there because that is a too strong
statement as find_lock_task_mm might not find any mm and so we still
could end up looping on the oom victim if it gets stuck somewhere in
__mmput. This particular patch didn't aim at closing that case. Plugging
that hole is planned later after the next upcoming merge window closes.
"
In order to help a forward progress for the OOM killer, make sure
that this really rare cases will not get into the way and hide
the mm from the oom killer by setting MMF_OOM_REAPED flag for it.
oom_scan_process_thread will ignore any TIF_MEMDIE task if it has
MMF_OOM_REAPED flag set to catch these oom victims.
After this patch we should guarantee a forward progress for the OOM
killer even when the selected victim is sharing memory with a kernel
thread or global init as long as the victims mm is still alive.
"
>
> Changes since v1
> - do not exit_oom_victim because oom_scan_process_thread will handle
> those which couldn't terminate in time. exit_oom_victim is not safe
> wrt. oom_disable synchronization.
>
> Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> mm/oom_kill.c | 25 +++++++++++++++++++++----
> 1 file changed, 21 insertions(+), 4 deletions(-)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index bfddc93ccd34..4c21f744daa6 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -283,10 +283,22 @@ enum oom_scan_t oom_scan_process_thread(struct oom_control *oc,
>
> /*
> * This task already has access to memory reserves and is being killed.
> - * Don't allow any other task to have access to the reserves.
> + * Don't allow any other task to have access to the reserves unless
> + * the task has MMF_OOM_REAPED because chances that it would release
> + * any memory is quite low.
> */
> - if (!is_sysrq_oom(oc) && atomic_read(&task->signal->oom_victims))
> - return OOM_SCAN_ABORT;
> + if (!is_sysrq_oom(oc) && atomic_read(&task->signal->oom_victims)) {
> + struct task_struct *p = find_lock_task_mm(task);
> + enum oom_scan_t ret = OOM_SCAN_ABORT;
> +
> + if (p) {
> + if (test_bit(MMF_OOM_REAPED, &p->mm->flags))
> + ret = OOM_SCAN_CONTINUE;
> + task_unlock(p);
> + }
> +
> + return ret;
> + }
>
> /*
> * If task is allocating a lot of memory and has been marked to be
> @@ -913,9 +925,14 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
> /*
> * We cannot use oom_reaper for the mm shared by this
> * process because it wouldn't get killed and so the
> - * memory might be still used.
> + * memory might be still used. Hide the mm from the oom
> + * killer to guarantee OOM forward progress.
> */
> can_oom_reap = false;
> + set_bit(MMF_OOM_REAPED, &mm->flags);
> + 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);
> continue;
> }
> do_send_sig_info(SIGKILL, SEND_SIG_FORCED, p, true);
> --
> 2.8.1
>
> --
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--
Michal Hocko
SUSE Labs
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