Re: Ext4 fiemap implementation

[Eric Sandeen <sandeen@sandeen.net>]

On 6/1/18 7:36 AM, Carlos Maiolino wrote:
> 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.

I think the fiemap docs[1] explicitly state that ext4's behavior is valid:

> Extents returned mirror
> those on disk - that is, the logical offset of the 1st returned extent
> may start before fm_start, and the range covered by the last returned
> extent may end after fm_length.

so if you're going to use this, you will need to work out //your// block
based on the overlapping extent that gets returned, just like userspace would
do.

-Eric

[1] Documentation/filesystems/fiemap.txt