Re: [PATCH RESEND x3 v9 1/9] iov_iter: add copy_struct_from_iter()

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On Mon, Jun 21, 2021 at 12:33:17PM -0700, Linus Torvalds wrote:
> On Mon, Jun 21, 2021 at 11:46 AM Omar Sandoval <osandov@xxxxxxxxxxx> wrote:
> >
> > 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@xxxxxxxxxxxxxxxxxxx/

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.



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