linux-kernel - Re: why do we need vmalloc_sync

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Date:	Mon, 15 Jun 2015 13:48:23 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andi Kleen <andi@...stfloor.org>, Oleg Nesterov <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: why do we need vmalloc_sync_all?

On Mon, Jun 15, 2015 at 1:28 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Andy Lutomirski <luto@...capital.net> wrote:
>
>> On Sun, Jun 14, 2015 at 7:47 PM, Andi Kleen <andi@...stfloor.org> wrote:
>> > Oleg Nesterov <oleg@...hat.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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