linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Fri, 21 Nov 2014 11:34:03 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Andy Lutomirski <luto@...capital.net>, Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Don Zickus <dzickus@...hat.com>, Dave Jones <davej@...hat.com>,
	"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: frequent lockups in 3.18rc4

On Fri, Nov 21, 2014 at 11:23 AM, Steven Rostedt <rostedt@...dmis.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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