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

[Andrew Morton <akpm@linux-foundation.org>]

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