linux-kernel - Re: [rfc] increase struct page size?!

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Message-ID: <20070519012235.GA15569@wotan.suse.de>
Date:	Sat, 19 May 2007 03:22:35 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-arch@...r.kernel.org
Subject: Re: [rfc] increase struct page size?!

On Fri, May 18, 2007 at 04:42:10PM +0100, Hugh Dickins wrote:
> On Fri, 18 May 2007, Nick Piggin wrote:
> > 
> > If we add 8 bytes to struct page on 64-bit machines, it becomes 64 bytes,
> > which is quite a nice number for cache purposes.
> > 
> > However we don't have to let those 8 bytes go to waste: we can use them
> > to store the virtual address of the page, which kind of makes sense for
> > 64-bit, because they can likely to use complicated memory models.
> 
> Sooner rather than later, don't we need those 8 bytes to expand from
> atomic_t to atomic64_t _count and _mapcount?  Not that we really need
> all 64 bits of both, but I don't know how to work atomically with less.

Yeah, that would be a very good use of it.

 
> (Why do I have this sneaking feeling that you're actually wanting
> to stick something into the lower bits of page->virtual?)

No, I just thought I would get even more flamed if I was just adding
padding that wasn't even doing _anything_ :) I remembered Ken had a
benchmark where page_address was slow, and the rest is history...
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