Re: FIEMAP ioctl gets "wrong" address for the extent

[David Sterba <dsterba@suse.cz>]

On Tue, Jul 07, 2020 at 05:43:09PM +0800, Anand Jain wrote:
> On 2/7/20 7:43 pm, David Sterba wrote:
> > On Thu, Jul 02, 2020 at 09:11:20AM +0000, Rebraca Dejan (BSOT/PJ-ES1-Bg) wrote:
> >> Hi all,
> >>
> >> I'm collecting file extents for our application from BtrFs filesystem image.
> >> I've noticed that for some files a get the "wrong" physical offset for
> >> start of the extent. I verified it using hexdump of the filesystem
> >> image: when dump the content starting from the address returned from
> >> FIEMAP ioctl, I see that the content is absolutely different from the
> >> content of the file itself. Also, the FIEMAP ioctl reports regular
> >> extent, it is not inline.
> > 
> > There are 3 address spaces:
> > 
> > - device physical offsets
> > - filesystem physical offsets
> > - filesystem logical offsets
> > 
> > What you seem to expect is that device physical and filesystem physical
> > and the same. This is not true in general in btrfs and fiemap will
> > return only the filesystem offsets. To get to the device offsets you'd
> > need to do the reverse mapping.
> 
> Do you think is it a good idea to rather update vfs? A quick check 
> indicates struct fiemap_extent has reserved space to hold the devid, and 
> should handle the backward compatibility issues.

This was proposed a few years back on LSF/MM, whether to extend fiemap
with the device related information or to add a completely new ioctl
that would not have to extend the existing interface in a way that could
become unwieldy.