Re: [PATCH v3 4/4] io_uring: add support for zone-append

[Matthew Wilcox]

On Mon, Jul 06, 2020 at 08:27:17AM -0600, Jens Axboe wrote:
> On 7/6/20 8:10 AM, Matthew Wilcox wrote:
> > On Sun, Jul 05, 2020 at 03:12:50PM -0600, Jens Axboe wrote:
> >> On 7/5/20 3:09 PM, Matthew Wilcox wrote:
> >>> On Sun, Jul 05, 2020 at 03:00:47PM -0600, Jens Axboe wrote:
> >>>> On 7/5/20 12:47 PM, Kanchan Joshi wrote:
> >>>>> From: Selvakumar S <selvakuma.s1@xxxxxxxxxxx>
> >>>>>
> >>>>> For zone-append, block-layer will return zone-relative offset via ret2
> >>>>> of ki_complete interface. Make changes to collect it, and send to
> >>>>> user-space using cqe->flags.
> > 
> >>> I'm surprised you aren't more upset by the abuse of cqe->flags for the
> >>> address.
> >>
> >> Yeah, it's not great either, but we have less leeway there in terms of
> >> how much space is available to pass back extra data.
> >>
> >>> What do you think to my idea of interpreting the user_data as being a
> >>> pointer to somewhere to store the address?  Obviously other things
> >>> can be stored after the address in the user_data.
> >>
> >> I don't like that at all, as all other commands just pass user_data
> >> through. This means the application would have to treat this very
> >> differently, and potentially not have a way to store any data for
> >> locating the original command on the user side.
> > 
> > I think you misunderstood me.  You seem to have thought I meant
> > "use the user_data field to return the address" when I actually meant
> > "interpret the user_data field as a pointer to where userspace
> > wants the address stored".
> 
> It's still somewhat weird to have user_data have special meaning, you're
> now having the kernel interpret it while every other command it's just
> an opaque that is passed through.
> 
> But it could of course work, and the app could embed the necessary
> u32/u64 in some other structure that's persistent across IO. If it
> doesn't have that, then it'd need to now have one allocated and freed
> across the lifetime of the IO.
> 
> If we're going that route, it'd be better to define the write such that
> you're passing in the necessary information upfront. In syscall terms,
> then that'd be something ala:
> 
> ssize_t my_append_write(int fd, const struct iovec *iov, int iovcnt,
> 			off_t *offset, int flags);
> 
> where *offset is copied out when the write completes. That removes the
> need to abuse user_data, with just providing the storage pointer for the
> offset upfront.

That works for me!  In io_uring terms, would you like to see that done
as adding:

        union {
                __u64   off;    /* offset into file */
+		__u64   *offp;	/* appending writes */
                __u64   addr2;
        };