Re: [PATCH 2/4 v3] fiemap: add EXTENT_DATA_COMPRESSED flag

[Christoph Hellwig <hch@infradead.org>]

On Fri, Dec 13, 2013 at 10:24:43AM +1100, Dave Chinner wrote:
> I'd prefer to just see the new physical length field always filled
> out, regardless of whether it is a compressed extent or not. In
> terms of backwards compatibility to userspace, it makes no
> difference because the value of reserved/unused fields is undefined
> by the API. Yes, the implementation zeros them, but there's nothing
> in the documentation that says "reserved fields must be zero".
> Hence I think we should just set it for every extent.
> 
> >From the point of view of the kernel API (fiemap_fill_next_extent),
> passing the physical extent size in the "len" parameter for normal
> extents, then passing 0 for the "physical length" makes absolutely
> no sense.

I tend to agree, but the additional complication here is that this is
a change to an existing API.  We'd need another HAVE_PHYS_LEN flag for
non-compressed extents so that userspace can rely on it.  Given that
it's only useful for that case I think the userspace API introduced
is the best we can get.

I think however that we should always pass the phys_len argument in the
kernel API just to make it less confusing.