Usefulness of SEEK_HOLE / SEEK_DATA in generic_file_llseek()

[Jan Kara <jack@suse.cz>]

  Hello,

  so I've now hit a xfstests failure for UDF which is caused by the
implementation of SEEK_HOLE / SEEK_DATA in generic_file_llseek(). UDF uses
that function as its .llseek method but it supports holes as any other unix
filesystem (e.g. ext2). The test in xfstests assumes that when it creates a
file by pwrite(fd, buf, bufsz, off), then SEEK_DATA on offset 0 should
return 'off' (off is reasonably rounded) but that's not true for the
implementation in generic_file_llseek().

Now I'm not so much interested in that test itself - that can be tweaked to
detect that case. But I rather wanted to ask - how useful is it to
implement SEEK_HOLE / SEEK_DATA the way it is in generic_file_llseek()?
Because it seems to me that any serious user will have to detect whether
SEEK_HOLE / SEEK_DATA works reasonably and if not, fall back to some
heuristic anyway. So why bother inventing bogus values in
generic_file_llseek and thus making detection of working implementation
harder?

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR