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

[Anand Jain <anand.jain@oracle.com>]

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.

struct fiemap_extent {
	__u64	fe_logical; /* logical offset in bytes for the start of
  * the extent */
	__u64	fe_physical; /* physical offset in bytes for the start
  * of the extent */
	__u64	fe_length; /* length in bytes for the extent */
	__u64	fe_reserved64[2];
	__u32	fe_flags; /* FIEMAP_EXTENT_* flags for this extent */
	__u32	fe_reserved[3];
};