On Tue, Jul 07, 2020 at 08:41:05PM +0530, Kanchan Joshi wrote:

On Mon, Jul 06, 2020 at 03:32:08PM +0100, Matthew Wilcox wrote:

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On Mon, Jul 06, 2020 at 08:27:17AM -0600, Jens Axboe wrote:

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On 7/6/20 8:10 AM, Matthew Wilcox wrote:

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On Sun, Jul 05, 2020 at 03:12:50PM -0600, Jens Axboe wrote:

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On 7/5/20 3:09 PM, Matthew Wilcox wrote:

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On Sun, Jul 05, 2020 at 03:00:47PM -0600, Jens Axboe wrote:

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On 7/5/20 12:47 PM, Kanchan Joshi wrote:

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From: Selvakumar S <redacted>

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.

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I'm surprised you aren't more upset by the abuse of cqe->flags for the
address.

Documentation (https://kernel.dk/io_uring.pdf) mentioned cqe->flags can carry
the metadata for the operation. I wonder if this should be called abuse.

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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.

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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;
       };

But there are peformance implications of this approach?
If I got it right, the workflow is: - Application allocates 64bit of space,
writes "off" into it and pass it
 in the sqe->addr2
- Kernel first reads sqe->addr2, reads the value to know the intended
 write-location, and stores the address somewhere (?) to be used during
 completion. Storing this address seems tricky as this may add one more
 cacheline (in io_kiocb->rw)?

io_kiocb is:
        /* size: 232, cachelines: 4, members: 19 */
        /* forced alignments: 1 */
        /* last cacheline: 40 bytes */
so we have another 24 bytes before io_kiocb takes up another cacheline.
If that's a serious problem, I have an idea about how to shrink struct
kiocb by 8 bytes so struct io_rw would have space to store another
pointer.

- During completion cqe res/flags are written as before, but extra step
 to copy the append-completion-result into that user-space address.
Extra steps are due to the pointer indirection.

... we've just done an I/O.  Concern about an extra pointer access
seems misplaced?

And it seems application needs to be careful about managing this 64bit of
space for a cluster of writes, especially if it wants to reuse the sqe
before the completion.
New one can handle 64bit result cleanly, but seems slower than current
one.

But userspace has to _do_ something with that information anyway.  So
it must already have somewhere to put that information.

I do think that interpretation of that field should be a separate flag
from WRITE_APPEND so apps which genuinely don't care about where the I/O
ended up don't have to allocate some temporary storage.  eg a logging
application which just needs to know that it managed to append to the
end of the log and doesn't need to do anything if it's successful.