linux-kernel - Re: [PATCH] mm: make read-only accessors take const parameters

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Message-ID: <20110415160701.GE7112@esdhcp04044.research.nokia.com>
Date:	Fri, 15 Apr 2011 19:07:01 +0300
From:	Phil Carmody <ext-phil.2.carmody@...ia.com>
To:	ext Christoph Lameter <cl@...ux.com>
Cc:	akpm@...ux-foundation.org, aarcange@...hat.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: make read-only accessors take const parameters

On 15/04/11 09:51 -0500, ext Christoph Lameter wrote:
> On Fri, 15 Apr 2011, Phil Carmody wrote:
> 
> > +++ b/include/linux/mm.h
> > @@ -353,9 +353,16 @@ static inline struct page *compound_head(struct page *page)
> >  	return page;
> >  }
> >
> > -static inline int page_count(struct page *page)
> > +static inline const struct page *compound_head_ro(const struct page *page)
> >  {
> > -	return atomic_read(&compound_head(page)->_count);
> > +	if (unlikely(PageTail(page)))
> > +		return page->first_page;
> > +	return page;
> > +}
> 
> Can you make compound_head take a const pointer too to avoid this?

Not in C, alas. As it returns what it's given I wouldn't want it to lie
about the type of what it returns, and some of its clients want it to
return something writeable.

The simplest macro would have multiple-evaluation issues:

#define compound_head(page) (PageTail(page) ? (page)->first_page : (page))

Not that there are any clients who would misuse that currently, but setting
traps isn't a good way to make things cleaner.

Phil
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