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

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Date:	Thu, 8 May 2008 08:27:47 +0300
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"Andrea Arcangeli" <andrea@...ranet.com>
Cc:	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"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, May 8, 2008 at 8:20 AM, Andrea Arcangeli <andrea@...ranet.com> wrote:
>  Actually I looked both at the struct and at the slab alignment just in
>  case it was changed recently. Now after reading your mail I also
>  compiled it just in case.
>
>  @@ -27,6 +27,7 @@ struct anon_vma {
>   struct anon_vma {
>
>         spinlock_t lock;        /* Serialize access to vma list */
>
>         struct list_head head;  /* List of private "related" vmas */
>  +       int flag:1;
>   };

You might want to read carefully what Linus wrote:

> The one that already has a 4 byte padding thing on x86-64 just after the
> spinlock? And that on 32-bit x86 (with less than 256 CPU's) would have two
> bytes of padding if we didn't just make the spinlock type unconditionally
> 32 bits rather than the 16 bits we actually _use_?

So you need to add the flag _after_ ->lock and _before_ ->head....

                        Pekka
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