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SubjectRe: [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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