Re: [PATCH v3 8/9] vfs: copy_file_range() can do a pagecache copy with splice

["Pádraig Brady" <P@draigBrady.com>]

On 25/09/15 21:48, Anna Schumaker wrote:
> The NFS server will need some kind offallback for filesystems that don't
> have any kind of copy acceleration, and it should be generally useful to
> have an in-kernel copy to avoid lots of switches between kernel and user
> space.
> 
> I make this configurable by adding two new flags.  Users who only want a
> reflink can pass COPY_FR_REFLINK, and users who want a full data copy can
> pass COPY_FR_COPY.  The default (flags=0) means to first attempt a
> reflink, but use the pagecache if that fails.
> 
> I moved the rw_verify_area() calls into the fallback code since some
> filesystems can handle reflinking a large range.
> 
> Signed-off-by: Anna Schumaker <Anna.Schumaker-ZwjVKphTwtPQT0dZR+AlfA@public.gmane.org>

Reviewed-by: Pádraig Brady <P@draigBrady.com>

LGTM. For my reference, for cp(1), mv(1), install(1), this will avoid
user space copies in the normal case, client side copies in the network
file system case, and provide a more generalized interface to reflink().

coreutils pseudo code is:

  unsigned int cfr_flags = COPY_FR_COPY;
  if (mode == mv)
    cfr_flags = 0; /* reflink falling back to normal */
  else if (mode == cp) {
    if --reflink || --reflink==always
      cfr_flags = COPY_FR_REFLINK;
    else if --reflink==auto
      cfr_flags = 0; /* reflink falling back to normal */
  }
  if vfs_copy_file_range(..., cfr_flags) == ENOTSUP
    normal_user_space_copy();

thanks,
Pádraig.