Re: task_numa_fault() && TASK_DEAD

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Sep 03, 2014 at 06:08:19PM +0200, Oleg Nesterov wrote:
> On 09/02, Oleg Nesterov wrote:
> >
> > The usage of TASK_DEAD in task_numa_fault() is wrong in any case.
> 
> Rik, I can't understand why task_numa_fault() needs this check at all,
> but "if (p->state == TASK_DEAD)" looks certainly wrong. You could replace
> this check with BUG_ON(p->state == TASK_DEAD). Perhaps you meant PF_EXITING?

Looking at 82727018b it appears the intent was to make sure we don't
re-create ->numa_fault after we free it. But you're right, we should
never get there with TASK_DEAD.

Also, given that task_numa_free() is called from __put_task_struct() I
tihnk we can safely delete this clause.

> And a stupid (really, I don't understand this code) question:
> 
> 	/* for example, ksmd faulting in a user's mm */
> 	if (!p->mm)
> 		return;

In general kernel threads have !->mm, and those cannot do the
accounting. The only way to get here is through get_user_pages() with
tsk != current and/or mm != current->mm.

> OK, but perhaps it make sense to pass "mm" as another argument and do
> 
> 	/* ksmd faulting in a user's mm, or debugger, or kthread use_mm() caller */
> 	if (p->mm != mm)
> 		return;
> 
> ?

I'm still somewhat fuzzy in the brain but that doesn't appear to
actually work, use_mm() explicitly sets ->mm so in that case it would
match just fine.

That said; I don't think we really need to worry about this. The !->mm
case is special in that that cannot ever work, the other cases are
extremely rare and will not skew accounting much if anything.