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From: Andy Lutomirski <luto@amacapital.net>
Date: Sun, 14 Jun 2015 19:57:46 -0700
Message-ID: <CALCETrUp5Xm1ZmzoSEGrq1D05myAUhCzNgXvv-Cga8xjEi-CeQ@mail.gmail.com>
Subject: Re: why do we need vmalloc_sync_all?
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To: Andi Kleen <andi@firstfloor.org>
Cc: Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, Andrew Morton <akpm@linux-foundation.org>, Denys Vlasenko <dvlasenk@redhat.com>, Brian Gerst <brgerst@gmail.com>, Peter Zijlstra <peterz@infradead.org>, Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>, Linus Torvalds <torvalds@linux-foundation.org>, Thomas Gleixner <tglx@linutronix.de>, Waiman Long <Waiman.Long@hp.com>

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.

--Andy

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