On Thu, Jun 24, 2021 at 07:41:12AM +0100, Christoph Hellwig wrote:

I'm also really worried with overloading the regular r/w path and
iov_iter with ever more special cases.  We already have various
performance problems in the path, and adding more special cases ain't
gonna help.

The changes to the normal path are:

* An extra check for RWF_ENCODED and FMODE_ENCODED_IO in kiocb_set_rw_flags().
* Splitting some of the checks in generic_write_checks() into a new
  function.
* Checks for the IOCB_ENCODED flag in the filesystem's
  read_iter/write_iter.

At least for Btrfs, the rest happens in a completely separate code path.
So, there are a couple of extra checks, but it's not as drastic as it
might first appear.