linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <CA+55aFy8gzquS-RnjxO3aax8=TNcrm42zK_udpOMdzxSjTbcQg@mail.gmail.com>
Date:	Wed, 19 Nov 2014 17:07:30 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	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 Wed, Nov 19, 2014 at 4:49 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> I thought so for a while, too, but now I disagree.  On PGE hardware,
> it seems entirely possible that the new stack would be in the TLB even
> if it's not visible via cr3.  Then, as soon as the TLB entry expires,
> we double-fault.

Ahh. Good point.

> I don't want vmalloc to avoid low-order allocations -- I want it to
> have guard pages.  The fact that a user-triggerable stack overflow is
> basically root right now and doesn't reliably OOPS scares me.

Well, if you do that, you would have to make the double-fault handler
aware of the stack issue anyway, and then you could just do teh same
PGD repopulation that a page fault does and return (for the case where
you didn't overflow the stack, just had the page tables unpopulated -
obviously an actual stack overflow should do something more drastic).

                   Linus
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