linux-kernel - Re: [RFC] fsblock

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Message-ID: <46863A23.2010001@garzik.org>
Date:	Sat, 30 Jun 2007 07:10:27 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC] fsblock

Christoph Hellwig wrote:
> On Sat, Jun 23, 2007 at 11:07:54PM -0400, Jeff Garzik wrote:
>>> - In line with the above item, filesystem block allocation is performed
>>>  before a page is dirtied. In the buffer layer, mmap writes can dirty a
>>>  page with no backing blocks which is a problem if the filesystem is
>>>  ENOSPC (patches exist for buffer.c for this).
>> This raises an eyebrow...  The handling of ENOSPC prior to mmap write is 
>> more an ABI behavior, so I don't see how this can be fixed with internal 
>> changes, yet without changing behavior currently exported to userland 
>> (and thus affecting code based on such assumptions).

> Not really, the current behaviour is a bug.  And it's not actually buffer
> layer specific - XFS now has a fix for that bug and it's generic enough
> that everyone could use it.

I'm not sure I follow.  If you require block allocation at mmap(2) time, 
rather than when a page is actually dirtied, you are denying userspace 
the ability to do sparse files with mmap.

A quick Google readily turns up people who have built upon the 
mmap-sparse-file assumption, and I don't think we want to break those 
assumptions as a "bug fix."

Where is the bug?

	Jeff


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