linux-kernel - Re: [PATCH 2/2] Optimize compound_head() by avoiding a shared page flag

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Message-Id: <20070407182506.4386dc13.akpm@linux-foundation.org>
Date:	Sat, 7 Apr 2007 18:25:06 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Hugh Dickins <hugh@...itas.com>, Nick Piggin <npiggin@...e.de>,
	dgc@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] Optimize compound_head() by avoiding a shared page
 flag

On Sat, 7 Apr 2007 17:21:38 -0700 (PDT) Christoph Lameter <clameter@....com> wrote:

> On Sat, 7 Apr 2007, Andrew Morton wrote:
> 
> > Which is all a ton of fun, but this subversion of the architecture's
> > freedom to use volatile, memory barriers etc is a worry.  We do the same in
> > page_alloc.c, of course...  
> 
> I just tried the approach that we discussed earlier and it was not 
> nice either.

We've discussed at least three approaches, so we don't know to what you refer.

>  Lets just use a page flag please.

Nope, try harder.

PageCompound is an unlikely case.  Back in the old days we would have done



	if (PageCompound(page))
		goto out_of_line;
back:

	do_stuff_with(page);
	return;

out_of_line:
	if (PageTail(page)) {
		page = page_tail(page);
		goto back;
	}
	<do other stuff>

and nowadays we hope that gcc does the above for us.  If it doesn't do it
for us, perhaps it needs open-coded help.

Because I don't expect there will be much efficiency difference between the
above and the use of another page flag.

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