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

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Date:	Sun, 31 Dec 2006 14:04:16 -0500 (EST)
From:	"Robert P. J. Day" <rpjday@...dspring.com>
To:	Paul Mundt <lethal@...ux-sh.org>
cc:	Arjan van de Ven <arjan@...radead.org>,
	Denis Vlasenko <vda.linux@...glemail.com>,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: replace "memset(...,0,PAGE_SIZE)" calls with "clear_page()"?

On Mon, 1 Jan 2007, Paul Mundt wrote:

> On Sat, Dec 30, 2006 at 06:04:14PM -0500, Robert P. J. Day wrote:
> > 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);
> >   ...
> >
> The problem with random grepping is that it doesn't give you any
> context. clear_page() isn't available in this case since we have a
> couple of different ways of implementing it, and the optimal
> approach is selected later on. There are also additional assumptions
> regarding alignment that don't allow clear_page() to be used
> directly as replacement for the memset() callsites (as has already
> been pointed out for some of the other architectures). While the
> empty_zero_page in this case sits on a full page boundary, others do
> not.
>
> You might find some places in drivers that do this where you might
> be able to optimize things slightly with a clear_page() (or
> copy_page() in the memcpy() case), but it's going to need a lot of
> manual auditing rather than a find and replace. Any sort of wins you
> get out of this would be marginal at best, anyways.
>
> The more interesting case would be page clustering/bulk page
> clearing with offload engines, and there's certainly room to build
> on the SGI patches for this.

your point is well taken -- i wasn't trying to suggest that a blind
cut-and-replace would be appropriate, only that there were an awful
lot of places where it wasn't clear that that kind of replacement
*wasn't* appropriate.  or perhaps even recommended.  (doing that kind
of search in the drivers/ directory would perhaps be more meaningful
than in the arch/ directory.  just my luck i picked a bad example.)

clearly, that kind of replacement might require manual intervention in
a lot of cases, no question.  as with other examples i've brought up
here, i'm just looking at this from a relatively newbie perspective,
where i'm perusing the code and, in this case, got to thinking, "gee,
given that every architecture defines a clear_page() macro, i wonder
why all these people keep calling memset()."  that's all.

kind of like how, given that include/linux/gfp.h defines the macro
__get_dma_pages(), so many people persist in calling
__get_free_pages() with a GFP_DMA setting.  that sort of thing.  :-)

rday
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