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Date: Fri, 21 Nov 2014 11:34:03 -0800
Message-ID: <CA+55aFw6w+pkG7fs7tBVm7jV_TjsnEyG5XDdk+Ghf5Af+ELUeA@mail.gmail.com>
Subject: Re: frequent lockups in 3.18rc4
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Andy Lutomirski <luto@amacapital.net>, Tejun Heo <tj@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Don Zickus <dzickus@redhat.com>, Dave Jones <davej@redhat.com>,
        "the arch/x86 maintainers" <x86@kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org

On Fri, Nov 21, 2014 at 11:23 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
>
> Should we at least check to see if it is present?
>
>         if (!(pgd_entry & 1))
>                 return -1;

Maybe. But what other entry could there be?

But yes, returning -1 is "safe", since it basically says "I'm not
doing a vmalloc thing, oops if this is a bad access". So that kind of
argues for being as aggressive as possible in returning 1.

So for the first one (!pgd_entry), instead of returning -1 only for a
completely empty entry, returning it for any non-present case is
probably right.

And for the second one (where we check whether there is anything at
all in the destination), returning -1 for "anything but zero" is
probably the right thing to do.

But in the end, if you have a corrupted top-level kernel page table,
it sounds to me like you're just royally screwed anyway. So I don't
think it matters *that* much.

So I kind of agree, but it wouldn't be my primary worry. My primary
worry is actually paravirt doing something insane.

                    Linus
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