linux-kernel - Re: [PATCH] zero_user_page uses in fs/buffer.c and fs/libfs.c

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Date:	Tue, 1 May 2007 21:09:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Nate Diller <nate.diller@...il.com>,
	Satyam Sharma <satyam.sharma@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] zero_user_page uses in fs/buffer.c and fs/libfs.c

On Tue, 1 May 2007 20:58:02 -0700 (PDT) Christoph Lameter <clameter@....com> wrote:

> On Tue, 1 May 2007, Nate Diller wrote:
> 
> > well, leave it to me to reply too quickly, sorry.  i think we should
> > leave simple_prepare_write() the way it is, since it's a library
> > function itself.  the other two callsites in your patch are buffers,
> > which may themselves be smaller than a page so you would need a
> > special function for just those two uses, there's no other way to
> > avoid making two calls to flush_dcache_page().  if it's tremendously
> > important to you to eliminate open coding of these, maybe make a
> > 'static int buffer_prepare_write()' or some such in fs/buffer.c
> 
> All three sites zap two parts of a page. If we had a 
> zero_user_page2 like this
> 
> zero_user_page2(page, start1, end1, start2, end2, kmap)
> 
> then all 3 sites could use the same funtion.
> 
> libfs.c:
> 
> zero_user_page_segments(page, 0, from, to, PAGE_CACHE_SIZE, KM_USER0);
> 
> buffer.c:
> 
> zero_user_page_segments(page, from, block_start, to, block_end, KM_USER0)
> 
> zero_user_page_segments(page, blockstart, from, to, block_end, KM_USER0)
> 

yup.  And perhaps zero_user_page() becomes a caller to
zero_user_page_segments() if we're sure that the compiler will dtrt.

> 
> I did not look through the whole kernel but this zapping segments is 
> likely frequent given the nature of the blocklayer.
> 
> The 3 call sites pretty ugly on their own. I think it would be good 
> to have one clearly commented version of this somewhere. Call sites
> will be much clearer since you do not have the kmap_ obfuscation nor
> the calculation of the length of each segment.

Sure.
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