Re: [PATCH] copy: adjust fiemap handling of sparse files

[Jim Meyering]

Pádraig Brady wrote:
...
> OK thanks for the info. So:
>
> XFS may miss some extents for sparse files before 2.6.36
> BTRFS and EXT4 need a sync before fiemap() before 2.6.38
> BTRFS can return overlapping extents before 2.6.38
>
> It seems like we should at least detect overlapping extents in our:
> http://git.sv.gnu.org/gitweb/?p=coreutils.git;a=blob;f=src/extent-scan.c;hb=HEAD
> and return false (don't use fiemap) in this case.
> We've already stopped doing the lseeks() but that assumes non overlapping 
> extents.
>
> So if we do the above we avoid corruption on BTRFS.
> We could also change to only enabling FIEMAP_FLAG_SYNC before 2.6.38?
> That still doesn't account for the XFS issue which was only fixed
> in a released kernel 5 months ago, and that leans towards
> us changing cp to just not use fiemap before 2.6.38 as you suggest.
> That allows us to remove the FIEMAP_FLAG_SYNC too.

Good summary.  That latter is simpler and gets my vote:
don't use FIEMAP with a pre-2.6.38 kernel