linux-kernel - Re: Linux 2.6.25-rc2

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Message-ID: <Pine.LNX.4.64.0802271532580.14380@schroedinger.engr.sgi.com>
Date:	Wed, 27 Feb 2008 15:34:07 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	Eric Dumazet <dada1@...mosbay.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Torsten Kaiser <just.for.lkml@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.25-rc2

On Tue, 19 Feb 2008, Mathieu Desnoyers wrote:

> I think you are right. A way to fix this would use the fact that the
> freelist is only useful to point to the first free object in a page. We
> could change it to an offset rather than an address.
> 
> The freelist would become a counter of type "long" which increments
> until it wraps at 2^32 or 2^64. A PAGE_MASK bitmask could then be used
> to get low order bits which would get the page offset of the first free
> object, while the high order bits would insure we can detect this type
> of object reuse when doing a cmpxchg. Upon free, the freelist counter
> should always be incremented; this would be provided by adding PAGE_SIZE
> to the counter and setting the LSBs to the correct offset.

Urgh.... That sounds way too complicated. Do you have an experimental 
patch that would allow us to see the impact?

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