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

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Message-ID: <Pine.LNX.4.64.0709211221560.17698@schroedinger.engr.sgi.com>
Date:	Fri, 21 Sep 2007 12:26:22 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	"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, 21 Sep 2007, 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).

The quicklists have been existing for a long time in this relationship 
and were actually developed initially (by Dave Miller I believe) to work 
this way. The generic TLB flushing thing may have developed in parallel on 
other platforms that did not use quicklists.

> 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.

Hmmmm.. Right the integration of the approaches that have now diverged on 
various platforms could be better.

> As it is, the quicklists try to be something separate, but by virtue of 
> being separate, they will always be buggy.

I guess we need to re-join what was separated by developments on different 
platforms.

> 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.

Right. But will that not mean that quicklists would have to be used on all 
platforms in a generic way?
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