linux-kernel - Re: replace "memset(...,0,PAGE_SIZE)" calls with "clear

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Message-ID: <Pine.LNX.4.64.0612301705250.16056@localhost.localdomain>
Date:	Sat, 30 Dec 2006 17:08:53 -0500 (EST)
From:	"Robert P. J. Day" <rpjday@...dspring.com>
To:	Denis Vlasenko <vda.linux@...glemail.com>
cc:	Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: replace "memset(...,0,PAGE_SIZE)" calls with "clear_page()"?

On Sat, 30 Dec 2006, Denis Vlasenko wrote:

> On Friday 29 December 2006 07:16, Robert P. J. Day wrote:
> >
> >   is there some reason there are so many calls of the form
> >
> >   memset(addr, 0, PAGE_SIZE)
> >
> > rather than the apparently equivalent invocation of
> >
> >   clear_page(addr)
> >
> > the majority of architectures appear to define the clear_page()
> > macro in their include/<arch>/page.h header file, but not entirely
> > identically, and in some cases that definition is conditional, as
> > with i386:
> >
> > =============================================================
> > #ifdef CONFIG_X86_USE_3DNOW
> > ...
> > #define clear_page(page)        mmx_clear_page((void *)(page))
> > ...
> > #else
> > ...
> > #define clear_page(page)        memset((void *)(page), 0, PAGE_SIZE)
> > ...
> > #endif
> > ============================================================
> >
> >   should it perhaps be part of the CodingStyle doc to use the
> > clear_page() macro rather than an explicit call to memset()?
> > (and should all architectures be required to define that macro?)
>
> clear_page assumes that given address is page aligned, I think. It
> may fail if you feed it with misaligned region's address.

i don't see how that can be true, given that most of the definitions
of the clear_page() macro are simply invocations of memset().  see for
yourself:

  $ grep -r "#define clear_page" include

my only point here was that lots of code seems to be calling memset()
when it would be clearer to invoke clear_page().  but there's still
something a bit curious happening here.  i'll poke around a bit more
before i ask, though.

rday
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