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

[Omar Sandoval <osandov@osandov.com>]

On Fri, Jun 18, 2021 at 07:42:41PM +0000, Al Viro wrote:
> On Fri, Jun 18, 2021 at 11:50:25AM -0700, Linus Torvalds wrote:
> 
> > I think that you may need it to be based on Al's series for that to
> > work, which might be inconvenient, though.
> > 
> > One other non-code issue: particularly since you only handle a subset
> > of the iov_iter cases, it would be nice to have an explanation for
> > _why_ those particular cases.
> > 
> > IOW, have some trivial explanation for each of the cases. "iovec" is
> > for regular read/write, what triggers the kvec and bvec cases?
> > 
> > But also, the other way around. Why doesn't the pipe case trigger? No
> > splice support?
> 
> Pipe ones are strictly destinations - they can't be sources.  So if you
> see it called for one of those, you've a bug.
> 
> Xarray ones are *not* - they can be sources, and that's missing here.

Ah, ITER_XARRAY was added recently so I missed it.

> Much more unpleasant, though, is that this thing has hard dependency on
> nr_seg == 1 *AND* openly suggests the use of iov_iter_single_seg_count(),
> which is completely wrong.  That sucker has some weird users left (as
> of #work.iov_iter), but all of them are actually due to API deficiencies
> and I very much hope to kill that thing off.
> 
> Why not simply add iov_iter_check_zeroes(), that would be called after
> copy_from_iter() and verified that all that's left in the iterator
> consists of zeroes?  Then this copy_struct_from_...() would be
> trivial to express through those two.  And check_zeroes would also
> be trivial, especially on top of #work.iov_iter.  With no calls of
> iov_iter_advance() at all, while we are at it...
> 
> IDGI... Omar, what semantics do you really want from that primitive?

RWF_ENCODED is intended to be used like this:

	struct encoded_iov encoded_iov = {
		/* compression metadata */ ...
	};
	char compressed_data[] = ...;
	struct iovec iov[] = {
		{ &encoded_iov, sizeof(encoded_iov) },
		{ compressed_data, sizeof(compressed_data) },
	};
	pwritev2(fd, iov, 2, -1, RWF_ENCODED);

Basically, we squirrel away the compression metadata in the first
element of the iovec array, and we use iov[0].iov_len so that we can
support future extensions of struct encoded_iov in the style of
copy_struct_from_user().

So this doesn't require nr_seg == 1. On the contrary, it's expected that
the rest of the iovec has the compressed payload. And to support the
copy_struct_from_user()-style versioning, we need to know the size of
the struct encoded_iov that userspace gave us, which is the reason for
the iov_iter_single_seg_count().

I know this interface isn't the prettiest. It started as a
Btrfs-specific ioctl, but this approach was suggested as a way to avoid
having a whole new I/O path:
https://lore.kernel.org/linux-fsdevel/20190905021012.GL7777@dread.disaster.area/
The copy_struct_from_iter() thing was proposed as a way to allow future
extensions here:
https://lore.kernel.org/linux-btrfs/20191022020215.csdwgi3ky27rfidf@yavin.dot.cyphar.com/

Please let me know if you have any suggestions for how to improve this.

Thanks,
Omar