linux-kernel - Re: A question on block_prepare

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Message-ID: <1287856846.1681.58.camel@leonhard>
Date:	Sun, 24 Oct 2010 03:00:46 +0900
From:	Namhyung Kim <namhyung@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: A question on block_prepare_write()

2010-10-23 (토), 10:40 -0700, Andrew Morton:
> On Sat, 23 Oct 2010 22:44:42 +0900 Namhyung Kim <namhyung@...il.com> wrote:
> 
> > Hello,
> > 
> > I see block_prepare_write() has local variable wait[2] to keep track of
> > buffer_heads which are not up-to-date. But I'm wondering how it could be
> > guaranteed there will be no more than 2 such buffer_heads? Is there any
> > restriction on the usage of the function? Using MAX_BUF_PER_PAGE instead
> > of the magic number 2 is just useless? I couldn't find any comments or
> > documentation on this.
> > 
> > Any of your comments would be greatly appreciated. TIA. :-)
> > 
> 
> block_prepare_write() may need to preread any buffer_head which are
> being only partially modified by the write().
> 
> Buffers which aren't being modified at all don't need to be preread. 
> Buffers which are being fully modified don't need to be preread
> (because all their data is being overwritten).
> 
> page:                      |-----------------------|
> buffer_heads:              |-----|-----|-----|-----|
> area we're writing to:               |---------|
> 
> There can only be a maximum of two partially-modified buffers in the page.

I see. It considers both edges of the to-be-written data. Thanks for
your clear and kindly explanation. :-)


-- 
Regards,
Namhyung Kim


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