Re: [PATCH 1/4] vfs: vfs-level fiemap interface

[Theodore Tso <tytso@mit.edu>]

On Tue, Oct 07, 2008 at 02:00:08PM +0100, Jamie Lokier wrote:
> Shared blocks must not be overwritten to the block device when
> unmounted - but they might be safe to *read*, e.g. by LILO.
> 
> Shared data is not the only situation where it's unsafe to overwrite
> blocks in place but they can be read: anything wich data-dependent
> metadata, such as checksums, data version numbers or indexing.
> 
>    FIEMAP_EXTENT_READ_ENCODED
>    FIEMAP_EXTENT_WRITE_ENCODED
> 

Um, sure, but are there potential users for FIEMAP that would want to
update a file in place while the filesystem is unmounted?  I've always
considered the flag (despite its many names) FIEMAP_EXTENT_ENCODED as
only giving permission to read from a particular block.  You're
correct that just because you can read from a particular block doesn't
mean that can safely update it in place.  But I never imagined that it
would be good and useful for the FIEMAP interface to give permission
to an application to be able to update a file in place while the
filesystem is unmounted.

If some application which wishes to use FIEMAP really wants to know
whether it's safe to write to an extent while the fs is unmounted, we
can add that flag later --- but I'm not that sure we really want to go
there.

Do you have a use in mind for this interface where this would be
useful?

Regards,

					- Ted