linux-kernel - Re: [GIT PULL] Lockless SLUB slowpaths for v3.1-rc1

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Date:	Sat, 30 Jul 2011 08:33:28 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	rientjes@...gle.com, hughd@...gle.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [GIT PULL] Lockless SLUB slowpaths for v3.1-rc1

Le vendredi 29 juillet 2011 à 16:18 -0700, Andi Kleen a écrit :
> Christoph Lameter <cl@...ux.com> writes:
> 
> > On Fri, 29 Jul 2011, Pekka Enberg wrote:
> >
> >> We haven't come up with a solution to keep struct page size the same but I
> >> think it's a reasonable trade-off.
> >
> > The change requires the page struct to be aligned to a double word
> > boundary. 
> 
> Why is that?
> 

Because cmpxchg16b is believed to require a 16bytes alignment.

http://siyobik.info/main/reference/instruction/CMPXCHG8B%2FCMPXCHG16B

64-Bit Mode Exceptions
...

#GP(0) 	If the memory address is in a non-canonical form. If memory
operand for CMPXCHG16B is not aligned on a 16-byte boundary. If
CPUID.01H:ECX.CMPXCHG16B[bit 13] = 0.



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