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://kernel.dk/io_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. > - 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.
