Re: [PATCH v1 02/11] VFS permit cross device vfs_copy_file_range

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Oct 19, 2018 at 11:30:18AM -0400, Olga Kornievskaia wrote:
> Allow copy_file_range to copy between different superblocks but only
> of the same file system types. This feature was of interest to CIFS
> as well as NFS.

> +	if (file_out->f_op->copy_file_range &&
> +			(file_in->f_op->copy_file_range ==
> +				file_out->f_op->copy_file_range)) {

That is seriously asking for trouble.  If at some point we add
a library function usable as ->copy_file_range() instance, this
will turn into a hard-to-spot problem.

Comparing methods like that is best avoided.  If you want to compare
fs types, do just that - it's not hard.

Another potential issue here is the interplay with local filesystems
using vfs_clone_file_prep_inodes() (or Darrick's series lifting that
into generic code).  There we assume the same block size on both
sides; that is automatically true if they live on the same superblock,
but with your changes it's no longer true.