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Re: why do we need vmalloc_sync_all?

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  Re: why do we need vmalloc_sync_all? Oleg Nesterov <oleg@redhat.com> - 2015-06-14 22:10 +0200
    Re: why do we need vmalloc_sync_all? Ingo Molnar <mingo@kernel.org> - 2015-06-15 22:30 +0200
      Re: why do we need vmalloc_sync_all? Andy Lutomirski <luto@amacapital.net> - 2015-06-15 22:50 +0200

#1164918 — Re: why do we need vmalloc_sync_all?

On 06/14, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@redhat.com> wrote:
>
> > I didn't read v2 yet, but I'd like to ask a question.
> >
> > Why do we need vmalloc_sync_all()?
> >
> > It has a single caller, register_die_notifier() which calls it without
> > any explanation. IMO, this needs a comment at least.
>
> Yes, it's used to work around crashes in modular callbacks: if the callbacks
> happens to be called from within the page fault path, before the vmalloc page
> fault handler runs, then we have a catch-22 problem.
>
> It's rare but not entirely impossible.

But again, the kernel no longer does this? do_page_fault() does vmalloc_fault()
without notify_die(). If it fails, I do not see how/why a modular DIE_OOPS
handler could try to resolve this problem and trigger another fault.

> > I am not sure I understand the changelog in 101f12af correctly, but at first
> > glance vmalloc_sync_all() is no longer needed at least on x86, do_page_fault()
> > no longer does notify_die(DIE_PAGE_FAULT). And btw DIE_PAGE_FAULT has no users.
> > DIE_MNI too...
> >
> > Perhaps we can simply kill it on x86?
>
> So in theory we could still have it run from DIE_OOPS, and that could turn a
> survivable kernel crash into a non-survivable one.

I don't understand... But OK, my understanding of this magic is very limited,
please forget.

Thanks,

Oleg.

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#1165537

* Andy Lutomirski <luto@amacapital.net> wrote:

> On Sun, Jun 14, 2015 at 7:47 PM, Andi Kleen <andi@firstfloor.org> wrote:
> > Oleg Nesterov <oleg@redhat.com> writes:
> >>
> >> But again, the kernel no longer does this? do_page_fault() does 
> >> vmalloc_fault() without notify_die(). If it fails, I do not see how/why a 
> >> modular DIE_OOPS handler could try to resolve this problem and trigger 
> >> another fault.
> >
> > The same problem can happen from NMI handlers or machine check handlers. It's 
> > not necessarily tied to page faults only.
> 
> AIUI, the point of the one and only vmalloc_sync_all call is to prevent 
> infinitely recursive faults when we call a notify_die callback.  The only thing 
> that it could realistically protect is module text or static non-per-cpu module 
> data, since that's the only thing that's reliably already in the init pgd.  I'm 
> with Oleg: I don't see how that can happen, since do_page_fault fixes up vmalloc 
> faults before it calls notify_die.

Yes, but what I meant is that it can happen if due to an unrelated kernel bug and 
unlucky timing we have installed this new handler just when that other unrelated 
kernel bug triggers: say a #GPF crash in kernel code.

In any case it should all be mooted with the removal of lazy PGD instantiation.

Thanks,

	Ingo
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#1165545

On Mon, Jun 15, 2015 at 1:28 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Andy Lutomirski <luto@amacapital.net> wrote:
>
>> On Sun, Jun 14, 2015 at 7:47 PM, Andi Kleen <andi@firstfloor.org> wrote:
>> > Oleg Nesterov <oleg@redhat.com> writes:
>> >>
>> >> But again, the kernel no longer does this? do_page_fault() does
>> >> vmalloc_fault() without notify_die(). If it fails, I do not see how/why a
>> >> modular DIE_OOPS handler could try to resolve this problem and trigger
>> >> another fault.
>> >
>> > The same problem can happen from NMI handlers or machine check handlers. It's
>> > not necessarily tied to page faults only.
>>
>> AIUI, the point of the one and only vmalloc_sync_all call is to prevent
>> infinitely recursive faults when we call a notify_die callback.  The only thing
>> that it could realistically protect is module text or static non-per-cpu module
>> data, since that's the only thing that's reliably already in the init pgd.  I'm
>> with Oleg: I don't see how that can happen, since do_page_fault fixes up vmalloc
>> faults before it calls notify_die.
>
> Yes, but what I meant is that it can happen if due to an unrelated kernel bug and
> unlucky timing we have installed this new handler just when that other unrelated
> kernel bug triggers: say a #GPF crash in kernel code.

I still don't see the problem.

CPU A: crash and start executing do_page_fault

CPU B: register_die_notifier

CPU A: notify_die

now we get a vmalloc fault, fix it up, and return to do_page_fault and
print the oops.

>
> In any case it should all be mooted with the removal of lazy PGD instantiation.

Agreed.

--Andy
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