Date: Sat, 30 Dec 2006 18:04:14 -0500 (EST)
From: "Robert P. J. Day" <rpjday@mindspring.com>
To: Arjan van de Ven <arjan@infradead.org>
cc: Denis Vlasenko <vda.linux@googlemail.com>,
       Linux kernel mailing list <linux-kernel@vger.kernel.org>
Subject: Re: replace "memset(...,0,PAGE_SIZE)" calls with "clear_page()"?
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On Sat, 30 Dec 2006, Arjan van de Ven wrote:

> rday wrote:

> > ... most of the definitions of the clear_page() macro are simply
> > invocations of memset().  see for yourself:

> *MOST*. Not all.

i did notice that.  while the majority of the architectures simply
define clear_page() as a macro calling memset(ptr, 0, PAGE_SIZE), the
rest will implement it in assembler code for whatever reason.  (i'm
assuming that *every* architecture *must* define clear_page() one way
or the other, is that correct?  that would seem fairly obvious, but
i just want to make sure i'm not missing anything obvious.)

> clear_page() is supposed to be for full real pages only... for
> example it allows the architecture to optimize for alignment, cache
> aliasing etc etc.

fair enough.  *technically*, not every call of the form
"memset(ptr,0,PAGE_SIZE)" necessarily represents an address that's on
a page boundary.  but, *realistically*, i'm guessing most of them do.
just grabbing a random example from some grep output:

arch/sh/mm/init.c:
  ...
  /* clear the zero-page */
  memset(empty_zero_page, 0, PAGE_SIZE);
  ...

my only point here is that, given that every architecture needs to
supply some kind of definition of a "clear_page()" routine, one would
think that *lots* of those memset() calls could reasonably be
rewritten as a clear_page() call for improved readibility, no?

and there are a *lot* of memset() calls like that.

rday
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