Re: [RFC] fsblock

[Jeff Garzik <jeff@garzik.org>]

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