linux-kernel - Re: [PATCH 08 of 11] anon-vma-rwsem

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Date:	Thu, 8 May 2008 09:11:33 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrea Arcangeli <andrea@...ranet.com>
cc:	Christoph Lameter <clameter@....com>,
	Andrew Morton <akpm@...ux-foundation.org>, steiner@....com,
	holt@....com, npiggin@...e.de, a.p.zijlstra@...llo.nl,
	kvm-devel@...ts.sourceforge.net, kanojsarcar@...oo.com,
	rdreier@...co.com, swise@...ngridcomputing.com,
	linux-kernel@...r.kernel.org, avi@...ranet.com, linux-mm@...ck.org,
	general@...ts.openfabrics.org, hugh@...itas.com,
	rusty@...tcorp.com.au, aliguori@...ibm.com, chrisw@...hat.com,
	marcelo@...ck.org, dada1@...mosbay.com, paulmck@...ibm.com
Subject: Re: [PATCH 08 of 11] anon-vma-rwsem

On Thu, 8 May 2008, Linus Torvalds wrote:
> 
> Also, we'd need to make it 
> 
> 	unsigned short flag:1;
> 
> _and_ change spinlock_types.h to make the spinlock size actually match the 
> required size (right now we make it an "unsigned int slock" even when we 
> actually only use 16 bits).

Btw, this is an issue only on 32-bit x86, because on 64-bit one we already 
have the padding due to the alignment of the 64-bit pointers in the 
list_head (so there's already empty space there).

On 32-bit, the alignment of list-head is obviously just 32 bits, so right 
now the structure is "perfectly packed" and doesn't have any empty space. 
But that's just because the spinlock is unnecessarily big.

(Of course, if anybody really uses NR_CPUS >= 256 on 32-bit x86, then the 
structure really will grow. That's a very odd configuration, though, and 
not one I feel we really need to care about).

			Linus
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