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

[Omar Sandoval <osandov@osandov.com>]

On Wed, Jun 23, 2021 at 08:06:39AM +1000, Dave Chinner wrote:
> On Mon, Jun 21, 2021 at 01:55:03PM -0700, Omar Sandoval wrote:
> > On Mon, Jun 21, 2021 at 01:46:04PM -0700, Omar Sandoval wrote:
> > > 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@osandov.com> 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@dread.disaster.area/
> > > 
> > > 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.
> > 
> > Oops, I dropped Dave from the Cc list at some point. Adding him back
> > now.
> 
> Fair summary. The only other thing that I'd add is this is an IO
> interface that requires issuing physical IO. So if someone wants
> high throughput for encoded IO, we really need AIO and/or io_uring
> support, and we get that for free if we use readv2/writev2
> interfaces.
> 
> Yes, it could be an ioctl() interface, but I think that this sort of
> functionality is exactly what extensible syscalls like
> preadv2/pwritev2 should be used for. It's a slight variant on normal
> IO, and that's exactly what the RWF_* flags are intended to be used
> for - allowing interesting per-IO variant behaviour without having
> to completely re-implemnt the IO path via custom ioctls every time
> we want slightly different functionality...

Al, Linus, what do you think? Is there a path forward for this series as
is? I'd be happy to have this functionality merged in any form, but I do
think that this approach with preadv2/pwritev2 using iov_len is decent
relative to the alternatives.