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

From: Omar Sandoval <osandov@osandov.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Kernel Team <kernel-team@fb.com>,
	Dave Chinner <dchinner@redhat.com>
Subject: Re: [PATCH RESEND x3 v9 1/9] iov_iter: add copy_struct_from_iter()
Date: Mon, 21 Jun 2021 13:55:03 -0700	[thread overview]
Message-ID: <YND8p7ioQRfoWTOU@relinquished.localdomain> (raw)
In-Reply-To: <YND6jOrku2JDgqjt@relinquished.localdomain>

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.