Re: [RFC v1 01/19] fs: Don't copy beyond the end of the file

["hch@infradead.org" <hch@infradead.org>]

[please trim your replies instead of this giant unreadable garbage,
 thanks!]

On Thu, Mar 09, 2017 at 10:29:48AM -0500, bfields@fieldses.org wrote:
> So I don't understand why it needed to be added to copy_file_range().
> The copy and clone semantics are different enough that I think callers
> want to know which they're getting.

Because if a file systems implements clone is literally is always better
than doing a copy loop, so using it is an absolute non-brainer.

> Don't we want it to have more or less the same behavior as a read-write
> loop?  People are probably running backup programs that depend on just
> simple copies, and maybe the results are good enough for their purposes,
> or maybe they're actually corrupting parts of their backups and don't
> know, but we can't suddenly start aborting their backups with errors and
> tell users it's for their own good.  So copy_file_range() callers will
> need to handle EINVAL on changing files somehow.

They do, and the system call has been in the tree for almost a year and
a half, so we can't simply change it.  Fortunately we do have a flags
argument that can be used to implement your preferred semantics if you
care deeply enough about them.