Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

Linus Torvalds
2016-06-23T17:44:10Z

On Thu, Jun 23, 2016 at 10:03 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>
> Let me quote my previous email ;)
>
> And we can't free/nullify it when the parent/debuger reaps a zombie,
> say, mark_oom_victim() expects that get_task_struct() protects
> thread_info as well.
>
> probably we can fix all such users though...

TIF_MEMDIE is indeed a potential problem, but I don't think
mark_oom_victim() is actually problematic.

mark_oom_victim() is called with either "current", or with a victim
that still has its mm and signal pointer (and the task is locked). So
the lifetime is already guaranteed - or that code is already very very
buggy, since it follows tsk->signal and tsk->mm

So as far as I can tell, that's all fine.

That said, by now it would actually in many ways be great if we could
get rid of thread_info entirely. The historical reasons for
thread_info have almost all been subsumed by the percpu area.

The reason for thread_info originally was

- we used to find the task_struct by just masking the stack pointer
(long long ago). When the task struct grew too big, we kept just the
critical pieces and some arch-specific stuff and , called it
"thread_info", and moved the rest to an external allocation and added
the pointer to it.

- the really crticial stuff we didn't want to follow a pointer for,
so things like preempt_count etc were in thread_info

- but they were *so* critical that PeterZ (at my prodding) moved
those things to percpu caches that get updated at schedule time
instead

so these days, thread_info has almost nothing really critical in it
any more. There's the thread-local flags, yes, but they could stay or
easily be moved to the task_struct or get similar per-cpu fixup as
preempt_count did a couple of years ago. The only annoyance is the few
remaining entry code assembly sequences, but I suspect they would
actually become simpler with a per-cpu thing, and with Andy's cleanups
they are pretty insignificant these days. There seems to be exactly
two uses of ASM_THREAD_INFO(TI_flags,.. left.

So I suspect that it would

(a) already be possible to just free the stack and thread info at
release time, because any rcu users will already be doing task_lock()
and check mm etc.

(b) it probably would be a nice cleanup to try to make it even more
obviously safe by just shrinking thread_info more (or even getting rid
of it entirely, but that may be too painful because there are other
architectures that may depend on it more).

I dunno. Looking at what remains of thread_info, it really doesn't
seem very critical.

The thread_info->tsk pointer, that was one of the most critical issues
and the main raison d'être of the thread_info, has been replaced on
x86 by just using the per-cpu "current_task". Yes,.there are probably
more than a few "ti->task" users left for legacy reasons, harking back
to when the thread-info was cheaper to access, but it shouldn't be a
big deal.

Linus