linux-kernel - Re: why does the macro "ZERO

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Message-ID: <Pine.LNX.4.64.0706070731250.14994@localhost.localdomain>
Date:	Thu, 7 Jun 2007 07:34:19 -0400 (EDT)
From:	"Robert P. J. Day" <rpjday@...dspring.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: why does the macro "ZERO_PAGE" take an argument?

On Thu, 7 Jun 2007, Nick Piggin wrote:

> Robert P. J. Day wrote:
> >   probably making a fool of myself here, but what is the purpose of
> > that single argument to the macro "ZERO_PAGE"?
> >
> > $ grep -r "define ZERO_PAGE" include
> > include/asm-frv/pgtable.h:#define ZERO_PAGE(vaddr)      ({ BUG(); NULL; })
> > include/asm-frv/pgtable.h:#define ZERO_PAGE(vaddr)
> > virt_to_page(empty_zero_page)
> > include/asm-v850/pgtable.h:#define ZERO_PAGE(vaddr)     ((void *)0x87654321)
> > include/asm-mips/pgtable.h:#define ZERO_PAGE(vaddr) \
> > include/asm-blackfin/pgtable.h:#define ZERO_PAGE(vaddr) (virt_to_page(0))
> > include/asm-parisc/pgtable.h:#define ZERO_PAGE(vaddr)
> > (virt_to_page(empty_zero_page))
> > include/asm-alpha/pgtable.h:#define ZERO_PAGE(vaddr)
> > (virt_to_page(ZERO_PGE))
> > ...
> >
> >   AFAICT, there are no definitions of that macro that actually use
> > that argument.  is that some kind of historical cruft?
>
> MIPS?

argh. that would be the *one* definition whose output got chopped
because of line continuation, and it would be only one that actually
uses the argument:

#define ZERO_PAGE(vaddr) \
        (virt_to_page((void *)(empty_zero_page + (((unsigned long)(vaddr)) & zero_page_mask))))

but it still leaves the question -- if ZERO_PAGE is meant to represent
a single, global shared page that is always zero, why would it *ever*
need to take an argument?  and what's so special about MIPS that it
differs from all the rest?

rday
-- 
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

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