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

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Message-ID: <a781481a0706070439p6dda152l37e475ae0fa698ef@mail.gmail.com>
Date:	Thu, 7 Jun 2007 17:09:59 +0530
From:	"Satyam Sharma" <satyam.sharma@...il.com>
To:	"Robert P. J. Day" <rpjday@...dspring.com>
Cc:	"Nick Piggin" <nickpiggin@...oo.com.au>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: why does the macro "ZERO_PAGE" take an argument?

On 6/7/07, Robert P. J. Day <rpjday@...dspring.com> wrote:
> 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?

The comment above empty_zero_page and zero_page_mask
declarations at arch/mips/mm/init.c:508 sheds light on this ...
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