Ext4 fiemap implementation

[Carlos Maiolino <cmaiolino@redhat.com>]

Hi,

I've been working on a patchset to get rid of the ->bmap infrastructure.

FIBMAP ioctl should be supported forever, so, basically I'm using Dave's idea to
use ->fiemap() implementation to handle FIBMAP ioctl, however, I've been facing
an issue with Ext4 FIEMAP in this case; basically:

When issuing a FIEMAP ioctl to Ext4with something like this:

fiemap->fm_start = block_num * blocksize;
fiemap->fm_length = 1;
fmap->fm_extent_count = 1;

I was expecting the fiemap_extent returned, to contain the physical block
from the logical request above, so:

physical block == fiemap_ext.fe_physical / blocksize


This works on XFS, which is using iomap_fiemap infrastructure. However, it
doesn't work on Ext4.

Ext4 always returns in fiemap_ext.fe_physical, the start of the extent, and not
the offset requested initially (if it lies somewhere beyond the first block in
the extent).

Ted, is there any restriction why ext4_fiemap isn't using iomap_fiemap()? Or any
reason why ext4 fiemap always returns the offset from the beginning of the
extent? Would you oppose to have it updated to return the offset initially
requested? Or maybe, change ext4_fiemap() to use iomap_fiemap()?

I read the fiemap documentation, but I didn't get a clear understanding if
fiemap should be returning the beginning of the extent, the offset initially
requested, or if it depends on FS implementation.

Cheers.




-- 
Carlos