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@xxxxxxxxxxxxxxxxxxx/
The copy_struct_from_iter() thing was proposed as a way to allow future
extensions here:
https://lore.kernel.org/linux-btrfs/20191022020215.csdwgi3ky27rfidf@xxxxxxxxxxxxxxxxxxxx/
Please let me know if you have any suggestions for how to improve this.
Thanks,
Omar
