linux-kernel - Re: [RFC][PATCH] page->mapping clarification [1/3] base functions

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Message-ID: <Pine.LNX.4.64.0709262015030.7064@blonde.wat.veritas.com>
Date:	Wed, 26 Sep 2007 20:31:02 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	clameter@....com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org, nickpiggin@...oo.com.au,
	ricknu-0@...dent.ltu.se, magnus.damm@...il.com
Subject: Re: [RFC][PATCH] page->mapping clarification [1/3] base functions

On Sat, 22 Sep 2007, KAMEZAWA Hiroyuki wrote:
> On Fri, 21 Sep 2007 18:02:47 +0100 (BST)
> Hugh Dickins <hugh@...itas.com> wrote:
> 
> > Or should I now leave PG_swapcache as is,
> > given your designs on page->mapping?
> > 
>  will conflict with my idea ?
> ==
> http://marc.info/?l=linux-mm&m=118956492926821&w=2
> ==

I asked because I had thought it would be a serious conflict: obviously
the patches as such would conflict quite a bit, but that's not serious,
one or the other just gets fixed up.

But now I don't see it - we both want to grab a further bit from the
low bits of the page->mapping pointer, you PAGE_MAPPING_INFO and me
PAGE_MAPPING_SWAP; but that's okay, so long as whoever is left using
bit (1<<2) is careful about the 32-bit case and remembers to put
__attribute__((aligned(sizeof(long long))))
on the declarations of struct address_space and struct anon_vma
and your struct page_mapping_info.

Would that waste a little memory?  I think not with SLUB,
but perhaps with SLOB, which packs a little tighter.

Hugh
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