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

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Date:	Mon, 9 Apr 2007 11:49:11 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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 Mon, 9 Apr 2007, Andrew Morton wrote:

> > The patch adds PageTail(page) and PageHead(page) to check if a page is the
> > head or the tail of a compound page. This is done by masking the two
> > bits describing the state of a compound page and then comparing them. So 
> > one comparision and a branch instead of two bit checks and two branches.
> > 
> 
> OK.  I'm still a bit concerned about bypassing the bitops synchronisation:
> barriers, volatile, etc.  We had lengthy ruminations on that a few years
> ago, I think when working on free_pages_check().

Those are important for bits that are changing while a page is in use. 
These are constant.

> > +#define __SetPageTail(page)	page->flags |= PG_head_tail_mask
> > +#define __ClearPageTail(page)	page->flags ~= PG_head_tail_mask
> 
> hm.  The lack of parenthesisation here _might_ be OK, but I haven't
> thought it through.

This is a command and not an expression. Ok will inline.
 
> And I'd prefer not to have to, because I know that the do { } while (0)
> thing works.  As do static inline functions.
> 
> > +#define PageHead(page)	((page->flags & PG_head_tail_mask) \
> > +				== (1L << PG_compound))
> > +#define __SetPageHead(page)	__SetCompoundPage(page)
> > +#define __ClearPageHead(page)	__ClearCompoundPage(page)
> 
> You meant __SetPageCompound and __ClearPageCompound.

Right.

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