linux-kernel - Re: x86_64: potential critical issue with quicklists and page table pages

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Date:	Tue, 25 Sep 2007 07:27:45 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Christoph Lameter <clameter@....com>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	akpm@...ux-foundation.org, ak@...e.de,
	linux-kernel@...r.kernel.org, tony.luck@...el.com,
	asit.k.mallick@...el.com
Subject: Re: x86_64: potential critical issue with quicklists and page
	table pages

On Fri, 2007-09-21 at 12:21 -0700, Linus Torvalds wrote:

> Yeah, and the whole thing seems totally bogus. It totally depends on 
> mmu_gather doing everything right (which very much includes the dependency 
> on mmu gathering disabling preempt).
> 
> For exmaple, if we were to go back to the original small tlb_gather with a 
> simple quicklist on the stack, rather than the per-cpu datastructure, the 
> quicklists would immediately break horribly - simply because they are 
> incorrectly now depending on the internal semantics of that tlb-gather.
> 
> As it is, the quicklists try to be something separate, but by virtue of 
> being separate, they will always be buggy.
> 
> The only way to fix it would be to integrate the quicklist stuff *with* 
> the mmu_gather stuff, so that these kinds of implementation issues are 
> explicitly shown in the relationship, instead of havign two "independent" 
> pieces of code where one piece very subtly depends on the exact 
> implementation of the other.

I've been mostly offline since KS (since a bit before in fact), so I
missed some of those discussions but so that you know, I'm toying a bit
with mmu gather and page table accessors at the moment, and one of the
things I've been contemplating is just that ... integrating a quicklist
in the gather to handle just that (and possibly other issues that have
been overlooked on some archs).

I'd suggest just reverting the patch for now (well, I see from the
commit list that you did just that) and I'll try to come up with
something better.

Christoph, I'd be happy if you didn't start butchering mmu_gather just
right now since I'm doing just that and it will collide all over the
place :-) Or if you want something specific done, please throw
ideas/patches at me and I'll integrate that in my serie.

Cheers,
Ben.

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