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

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Date:	Wed, 16 May 2007 23:04:11 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Chris Mason <chris.mason@...cle.com>
CC:	David Woodhouse <dwmw2@...radead.org>,
	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

Chris Mason wrote:
> 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.

I'll add to David's question about David's comment on David's patch, yes
it will be modified but in that case it would be zero-filled as Chris
says. However I believe this is incorrect behaviour.

It is possible to easily fix that so it would only happen via a tiny race
window (where the source memory gets unmapped at just the right time)
however nobody seemed to interested (just by checking the return value of
fault_in_pages_readable).

The buffered write patches I'm working on fix that (among other things) of
course. But they do away with prepare_write and introduce new aops, and
they indeed must not expect the full range to have been written to.

-- 
SUSE Labs, Novell Inc.
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