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Mon, 06 Jul 2020 07:33:40 -0700 (PDT) Subject: Re: [PATCH v3 4/4] io_uring: add support for zone-append To: Matthew Wilcox Cc: Kanchan Joshi , viro@zeniv.linux.org.uk, bcrl@kvack.org, hch@infradead.org, Damien.LeMoal@wdc.com, asml.silence@gmail.com, linux-fsdevel@vger.kernel.org, mb@lightnvm.io, linux-kernel@vger.kernel.org, linux-aio@kvack.org, io-uring@vger.kernel.org, linux-block@vger.kernel.org, Selvakumar S , Nitesh Shetty , Javier Gonzalez References: <1593974870-18919-1-git-send-email-joshi.k@samsung.com> <1593974870-18919-5-git-send-email-joshi.k@samsung.com> <20200705210947.GW25523@casper.infradead.org> <239ee322-9c38-c838-a5b2-216787ad2197@kernel.dk> <20200706141002.GZ25523@casper.infradead.org> <4a9bf73e-f3ee-4f06-7fad-b8f8861b0bc1@kernel.dk> <20200706143208.GA25523@casper.infradead.org> From: Jens Axboe Message-ID: Date: Mon, 6 Jul 2020 08:33:39 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <20200706143208.GA25523@casper.infradead.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 7/6/20 8:32 AM, 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 >>>>>>> >>>>>>> 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; > }; > Either that, or just use addr2 for it directly. I consider the appending writes a marginal enough use case that it doesn't really warrant adding a specially named field for that. -- Jens Axboe