On Sat, Jun 30, 2007 at 07:10:27AM -0400, Jeff Garzik wrote:

quoted

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?

It's not mmap time but page dirtying time.  Currently the default behaviour
is not to allocate at page dirtying time but rather at writeout time in
some scenarious.

(and s/allocation/reservation/ applies for delalloc of course)

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