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

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Message-ID: <Pine.LNX.4.64.0706070750540.16051@localhost.localdomain>
Date:	Thu, 7 Jun 2007 07:53:08 -0400 (EDT)
From:	"Robert P. J. Day" <rpjday@...dspring.com>
To:	Satyam Sharma <satyam.sharma@...il.com>
cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: why does the macro "ZERO_PAGE" take an argument?

On Thu, 7 Jun 2007, Satyam Sharma wrote:

> 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 ...

well, it *sort of* does.  at line 64 of that file:

/*
 * We have up to 8 empty zeroed pages so we can map one of the right colour
 * when needed.  This is necessary only on R4000 / R4400 SC and MC versions
 * where we have to avoid VCED / VECI exceptions for good performance at
 * any price.  Since page is never written to after the initialization we
 * don't have to care about aliases on other CPUs.
 */

although it's not clear where in the source tree are the invocations
that would actually make a difference to a MIPS system, which is why
i've CC'ed ralf on this.  i'm sure he can clear this up. :-)

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

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