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

[Kanchan Joshi]

On Tue, Jul 07, 2020 at 04:52:37PM +0100, Matthew Wilcox wrote:
> 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:
> > > 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.
> >
> > Documentation (https://protect2.fireeye.com/url?k=297dbcbf-74aee030-297c37f0-0cc47a31ce52-632d3561909b91fc&q=1&u=https%3A%2F%2Fkernel.dk%2Fio_uring.pdf) mentioned cqe->flags can carry
> > the metadata for the operation. I wonder if this should be called abuse.
> >
> > > > >> 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;
> > >        };
> > 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.
Yes, io_kiocb has room. Cache-locality wise whether that is fine or
it must be placed within io_rw - I'll come to know once I get to
implement this. Please share the idea you have, it can come handy.

> > - 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?

I was thinking about both read-from (submission) and write-to
(completion) from user-space pointer, and all those checks APIs
(get_user, copy_from_user) perform.....but when seen against I/O (that
too direct), it does look small. Down the line it may matter for cached-IO
but I get your point. 

> > 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.

Right. But it is part of SQE/CQE currently, and in new scheme it gets
decoupled and needs to be managed separately. 
> 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.
Would you want that new flag to do what RWF_APPEND does as well? 
In v2, we had a separate flag RWF_ZONE_APPEND and did not use
file-append infra at all. Thought was: apps using the new flag will
always look at where write landed.

In v3, I've been looking at this as: 
zone-append = file-append + something-extra-to-know-where-write-landed.
We see to it that something-extra does not get executed for regular
files/block-dev append (ref: FMODE_ZONE_APPEND patch1)....and existing
behavior (the one you described for logging application) is retained.
But on a zoned-device/file, file-append becomes zone-append, all the
time.