On Mon, Jun 21, 2021 at 12:33:17PM -0700, Linus Torvalds wrote:

On Mon, Jun 21, 2021 at 11:46 AM Omar Sandoval [off-list ref] wrote:

quoted

How do we get the userspace size with the encoded_iov.size approach?
We'd have to read the size from the iov_iter before writing to the rest
of the iov_iter. Is it okay to mix the iov_iter as a source and
destination like this? From what I can tell, it's not intended to be
used like this.

I guess it could work that way, but yes, it's ugly as hell. And I
really don't want a readv() system call - that should write to the
result buffer - to first have to read from it.

So I think the original "just make it be the first iov entry" is the
better approach, even if Al hates it.

Although I still get the feeling that using an ioctl is the *really*
correct way to go. That was my first reaction to the series
originally, and I still don't see why we'd have encoded data in a
regular read/write path.

What was the argument against ioctl's, again?

The suggestion came from Dave Chinner here:
https://lore.kernel.org/linux-fsdevel/20190905021012.GL7777@dread.disaster.area/ (local)

His objection to an ioctl was two-fold:

1. This interfaces looks really similar to normal read/write, so we
   should try to use the normal read/write interface for it. Perhaps
   this trouble with iov_iter has refuted that.
2. The last time we had Btrfs-specific ioctls that eventually became
   generic (FIDEDUPERANGE and FICLONE{,RANGE}), the generalization was
   painful. Part of the problem with clone/dedupe was that the Btrfs
   ioctls were underspecified. I think I've done a better job of
   documenting all of the semantics and corner cases for the encoded I/O
   interface (and if not, I can address this). The other part of the
   problem is that there were various sanity checks in the normal
   read/write paths that were missed or drifted out of sync in the
   ioctls. That requires some vigilance going forward. Maybe starting
   this off as a generic (not Btrfs-specific) ioctl right off the bat
   will help.

If we do go the ioctl route, then we also have to decide how much of
preadv2/pwritev2 it should emulate. Should it use the fd offset, or
should that be an ioctl argument? Some of the RWF_ flags would be useful
for encoded I/O, too (RWF_DSYNC, RWF_SYNC, RWF_APPEND), should it
support those? These bring us back to Dave's first point.