linux-kernel - Re: [PATCH 1 of 2] block_page

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Message-ID: <20070516125341.GS26766@think.oraclecorp.com>
Date:	Wed, 16 May 2007 08:53:41 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	David Howells <dhowells@...hat.com>, David Chinner <dgc@....com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 1 of 2] block_page_mkwrite() Implementation V2

On Wed, May 16, 2007 at 08:09:19PM +0800, David Woodhouse wrote:
> On Wed, 2007-05-16 at 11:19 +0100, David Howells wrote:
> > The start and end points passed to block_prepare_write() delimit the region of
> > the page that is going to be modified.  This means that prepare_write()
> > doesn't need to fill it in if the page is not up to date. 
> 
> Really? Is it _really_ going to be modified? Even if the pointer
> userspace gave to write() is bogus, and is going to fault half-way
> through the copy_from_user()?

This is why there are so many variations on copy_from_user that zero on
faults.  One way or another, the prepare_write/commit_write pair are
responsible for filling it in.

-chris

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