linux-kernel - Re: why do we need vmalloc_sync

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Date:	Sun, 14 Jun 2015 19:57:46 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	"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 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.

--Andy
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