On 05/30, Michal Hocko wrote:
>
> task_will_free_mem is rather weak.
I was thinking about the similar change because I noticed that try_oom_reaper()
is very, very wrong.
To the point I think that we need another change for stable which simply removes
spin_lock_irq(sighand->siglock) from try_oom_reaper(). It buys nothing, we can
check signal_group_exit() (which is wrong too ;) lockless, and at the same time
the kernel can crash because we can hit ->siglock == NULL.
So I do think this change is good in general.
I think that task_will_free_mem() should be un-inlined, and __task_will_free_mem()
should go into mm/oom-kill.c... but this is minor.
> -static inline bool task_will_free_mem(struct task_struct *task)
> +static inline bool __task_will_free_mem(struct task_struct *task)
> {
> struct signal_struct *sig = task->signal;
>
> @@ -119,16 +119,69 @@ static inline bool task_will_free_mem(struct task_struct *task)
> if (sig->flags & SIGNAL_GROUP_COREDUMP)
> return false;
>
> - if (!(task->flags & PF_EXITING))
> + if (!(task->flags & PF_EXITING || fatal_signal_pending(task)))
> return false;
>
> /* Make sure that the whole thread group is going down */
> - if (!thread_group_empty(task) && !(sig->flags & SIGNAL_GROUP_EXIT))
> + if (!thread_group_empty(task) &&
> + !(sig->flags & SIGNAL_GROUP_EXIT || fatal_signal_pending(task)))
> return false;
>
> return true;
> }
Well, let me suggest this again. I think it should do
if (SIGNAL_GROUP_COREDUMP)
return false;
if (SIGNAL_GROUP_EXIT)
return true;
if (thread_group_empty() && PF_EXITING)
return true;
return false;
we do not need fatal_signal_pending(), in this case SIGNAL_GROUP_EXIT should
be set (ignoring some bugs with sub-namespaces which we need to fix anyway).
At the same time, we do not want to return false if PF_EXITING is not set
if SIGNAL_GROUP_EXIT is set.
> +static inline bool task_will_free_mem(struct task_struct *task)
> +{
> + struct mm_struct *mm = NULL;
> + struct task_struct *p;
> + bool ret;
> +
> + /*
> + * If the process has passed exit_mm we have to skip it because
> + * we have lost a link to other tasks sharing this mm, we do not
> + * have anything to reap and the task might then get stuck waiting
> + * for parent as zombie and we do not want it to hold TIF_MEMDIE
> + */
> + p = find_lock_task_mm(task);
> + if (!p)
> + return false;
> +
> + if (!__task_will_free_mem(p)) {
> + task_unlock(p);
> + return false;
> + }
> +
> + mm = p->mm;
> + if (atomic_read(&mm->mm_users) <= 1) {
this is sub-optimal, we should probably take signal->live or ->nr_threads
into account... but OK, we can do this later.
> + rcu_read_lock();
> + for_each_process(p) {
> + ret = __task_will_free_mem(p);
> + if (!ret)
> + break;
> + }
> + rcu_read_unlock();
Yes, I agree very much.
But it seems you forgot to add the process_shares_mm() check into this loop?
and perhaps it also makes sense to add
if (same_thread_group(tsk, p))
continue;
This should not really matter, we know that __task_will_free_mem(p) should return
true. Just to make it more clear.
And. I think this needs smp_rmb() at the end of the loop (assuming we have the
process_shares_mm() check here). We need it to ensure that we read p->mm before
we read next_task(), to avoid the race with exit() + clone(CLONE_VM).
Oleg.
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