Re: [PATCH, RFC] xfs: re-enable FIBMAP on reflink; disable for swap

[Brian Foster <bfoster@redhat.com>]

On Thu, Aug 30, 2018 at 11:35:46AM -0500, Eric Sandeen wrote:
> On 8/30/18 11:36 AM, Christoph Hellwig wrote:
> > On Thu, Aug 30, 2018 at 11:31:40AM -0500, Eric Sandeen wrote:
> >> That's no reason to uniquely disallow it for reflinked files, though;
> >> the problem is universal.  It's true for fiemap as well.  So I'm not sure
> >> that's an argument against the patch?
> > 
> > fiemap at least tells you an extent is shared, bmap does not.
> 
> yes, so bmap is clearly the wrong interface to use if you want to
> write directly to a file's blocks.  But if you know enough to check
> the fiemap shared flag, you know enough to not use fibmap for that purpose...
> 

FWIW, this patch seems reasonable to me. To Christoph's point, I don't
think either interface really grants license to write to the underlying
blocks, so either way it's technically being abused for this purpose.
Unless there's a clear way to return an error for a particular type of
file, I think it's reasonable behavior for fibmap to expose the data it
supports (i.e., block maps) and drop the data it doesn't (reflink
state).

Brian

> -Eric