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Thu, 10 Sep 2020 05:40:17 -0700 (PDT) To: Jens Axboe , io-uring References: <8f6871c4-1344-8556-25a7-5c875aebe4a5@gmail.com> <622649c5-e30d-bc3c-4709-bbe60729cca1@kernel.dk> <1c088b17-53bb-0d6d-6573-a1958db88426@kernel.dk> <801ed334-54ea-bdee-4d81-34b7e358b506@gmail.com> <370c055e-fa8d-0b80-bd34-ba3ba9bc6b37@kernel.dk> <74c2802e-788e-d6b2-3ee6-5ef67950dc94@gmail.com> From: Pavel Begunkov Autocrypt: addr=asml.silence@gmail.com; prefer-encrypt=mutual; keydata= mQINBFmKBOQBEAC76ZFxLAKpDw0bKQ8CEiYJRGn8MHTUhURL02/7n1t0HkKQx2K1fCXClbps bdwSHrhOWdW61pmfMbDYbTj6ZvGRvhoLWfGkzujB2wjNcbNTXIoOzJEGISHaPf6E2IQx1ik9 6uqVkK1OMb7qRvKH0i7HYP4WJzYbEWVyLiAxUj611mC9tgd73oqZ2pLYzGTqF2j6a/obaqha +hXuWTvpDQXqcOZJXIW43atprH03G1tQs7VwR21Q1eq6Yvy2ESLdc38EqCszBfQRMmKy+cfp W3U9Mb1w0L680pXrONcnlDBCN7/sghGeMHjGKfNANjPc+0hzz3rApPxpoE7HC1uRiwC4et83 CKnncH1l7zgeBT9Oa3qEiBlaa1ZCBqrA4dY+z5fWJYjMpwI1SNp37RtF8fKXbKQg+JuUjAa9 Y6oXeyEvDHMyJYMcinl6xCqCBAXPHnHmawkMMgjr3BBRzODmMr+CPVvnYe7BFYfoajzqzq+h EyXSl3aBf0IDPTqSUrhbmjj5OEOYgRW5p+mdYtY1cXeK8copmd+fd/eTkghok5li58AojCba jRjp7zVOLOjDlpxxiKhuFmpV4yWNh5JJaTbwCRSd04sCcDNlJj+TehTr+o1QiORzc2t+N5iJ NbILft19Izdn8U39T5oWiynqa1qCLgbuFtnYx1HlUq/HvAm+kwARAQABtDFQYXZlbCBCZWd1 bmtvdiAoc2lsZW5jZSkgPGFzbWwuc2lsZW5jZUBnbWFpbC5jb20+iQJOBBMBCAA4FiEE+6Ju PTjTbx479o3OWt5b1Glr+6UFAlmKBOQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQ Wt5b1Glr+6WxZA//QueaKHzgdnOikJ7NA/Vq8FmhRlwgtP0+E+w93kL+ZGLzS/cUCIjn2f4Q Mcutj2Neg0CcYPX3b2nJiKr5Vn0rjJ/suiaOa1h1KzyNTOmxnsqE5fmxOf6C6x+NKE18I5Jy xzLQoktbdDVA7JfB1itt6iWSNoOTVcvFyvfe5ggy6FSCcP+m1RlR58XxVLH+qlAvxxOeEr/e aQfUzrs7gqdSd9zQGEZo0jtuBiB7k98t9y0oC9Jz0PJdvaj1NZUgtXG9pEtww3LdeXP/TkFl HBSxVflzeoFaj4UAuy8+uve7ya/ECNCc8kk0VYaEjoVrzJcYdKP583iRhOLlZA6HEmn/+Gh9 4orG67HNiJlbFiW3whxGizWsrtFNLsSP1YrEReYk9j1SoUHHzsu+ZtNfKuHIhK0sU07G1OPN 2rDLlzUWR9Jc22INAkhVHOogOcc5ajMGhgWcBJMLCoi219HlX69LIDu3Y34uIg9QPZIC2jwr 24W0kxmK6avJr7+n4o8m6sOJvhlumSp5TSNhRiKvAHB1I2JB8Q1yZCIPzx+w1ALxuoWiCdwV M/azguU42R17IuBzK0S3hPjXpEi2sK/k4pEPnHVUv9Cu09HCNnd6BRfFGjo8M9kZvw360gC1 reeMdqGjwQ68o9x0R7NBRrtUOh48TDLXCANAg97wjPoy37dQE7e5Ag0EWYoE5AEQAMWS+aBV IJtCjwtfCOV98NamFpDEjBMrCAfLm7wZlmXy5I6o7nzzCxEw06P2rhzp1hIqkaab1kHySU7g dkpjmQ7Jjlrf6KdMP87mC/Hx4+zgVCkTQCKkIxNE76Ff3O9uTvkWCspSh9J0qPYyCaVta2D1 Sq5HZ8WFcap71iVO1f2/FEHKJNz/YTSOS/W7dxJdXl2eoj3gYX2UZNfoaVv8OXKaWslZlgqN jSg9wsTv1K73AnQKt4fFhscN9YFxhtgD/SQuOldE5Ws4UlJoaFX/yCoJL3ky2kC0WFngzwRF Yo6u/KON/o28yyP+alYRMBrN0Dm60FuVSIFafSqXoJTIjSZ6olbEoT0u17Rag8BxnxryMrgR dkccq272MaSS0eOC9K2rtvxzddohRFPcy/8bkX+t2iukTDz75KSTKO+chce62Xxdg62dpkZX xK+HeDCZ7gRNZvAbDETr6XI63hPKi891GeZqvqQVYR8e+V2725w+H1iv3THiB1tx4L2bXZDI DtMKQ5D2RvCHNdPNcZeldEoJwKoA60yg6tuUquvsLvfCwtrmVI2rL2djYxRfGNmFMrUDN1Xq F3xozA91q3iZd9OYi9G+M/OA01husBdcIzj1hu0aL+MGg4Gqk6XwjoSxVd4YT41kTU7Kk+/I 5/Nf+i88ULt6HanBYcY/+Daeo/XFABEBAAGJAjYEGAEIACAWIQT7om49ONNvHjv2jc5a3lvU aWv7pQUCWYoE5AIbDAAKCRBa3lvUaWv7pfmcEACKTRQ28b1y5ztKuLdLr79+T+LwZKHjX++P 4wKjEOECCcB6KCv3hP+J2GCXDOPZvdg/ZYZafqP68Yy8AZqkfa4qPYHmIdpODtRzZSL48kM8 LRzV8Rl7J3ItvzdBRxf4T/Zseu5U6ELiQdCUkPGsJcPIJkgPjO2ROG/ZtYa9DvnShNWPlp+R uPwPccEQPWO/NP4fJl2zwC6byjljZhW5kxYswGMLBwb5cDUZAisIukyAa8Xshdan6C2RZcNs rB3L7vsg/R8UCehxOH0C+NypG2GqjVejNZsc7bgV49EOVltS+GmGyY+moIzxsuLmT93rqyII 5rSbbcTLe6KBYcs24XEoo49Zm9oDA3jYvNpeYD8rDcnNbuZh9kTgBwFN41JHOPv0W2FEEWqe JsCwQdcOQ56rtezdCJUYmRAt3BsfjN3Jn3N6rpodi4Dkdli8HylM5iq4ooeb5VkQ7UZxbCWt UVMKkOCdFhutRmYp0mbv2e87IK4erwNHQRkHUkzbsuym8RVpAZbLzLPIYK/J3RTErL6Z99N2 m3J6pjwSJY/zNwuFPs9zGEnRO4g0BUbwGdbuvDzaq6/3OJLKohr5eLXNU3JkT+3HezydWm3W OPhauth7W0db74Qd49HXK0xe/aPrK+Cp+kU1HRactyNtF8jZQbhMCC8vMGukZtWaAwpjWiiH bA== Subject: Re: [PATCH for-next] io_uring: ensure IOSQE_ASYNC file table grabbing works, with SQPOLL Message-ID: <33a6730c-8e0c-e34f-9094-c256a13961cd@gmail.com> Date: Thu, 10 Sep 2020 15:37:20 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 09/09/2020 19:07, Jens Axboe wrote: > On 9/9/20 9:48 AM, Pavel Begunkov wrote: >> On 09/09/2020 16:10, Jens Axboe wrote: >>> On 9/9/20 1:09 AM, Pavel Begunkov wrote: >>>> On 09/09/2020 01:54, Jens Axboe wrote: >>>>> On 9/8/20 3:22 PM, Jens Axboe wrote: >>>>>> On 9/8/20 2:58 PM, Pavel Begunkov wrote: >>>>>>> On 08/09/2020 20:48, Jens Axboe wrote: >>>>>>>> Fd instantiating commands like IORING_OP_ACCEPT now work with SQPOLL, but >>>>>>>> we have an error in grabbing that if IOSQE_ASYNC is set. Ensure we assign >>>>>>>> the ring fd/file appropriately so we can defer grab them. >>>>>>> >>>>>>> IIRC, for fcheck() in io_grab_files() to work it should be under fdget(), >>>>>>> that isn't the case with SQPOLL threads. Am I mistaken? >>>>>>> >>>>>>> And it looks strange that the following snippet will effectively disable >>>>>>> such requests. >>>>>>> >>>>>>> fd = dup(ring_fd) >>>>>>> close(ring_fd) >>>>>>> ring_fd = fd >>>>>> >>>>>> Not disagreeing with that, I think my initial posting made it clear >>>>>> it was a hack. Just piled it in there for easier testing in terms >>>>>> of functionality. >>>>>> >>>>>> But the next question is how to do this right...> >>>>> Looking at this a bit more, and I don't necessarily think there's a >>>>> better option. If you dup+close, then it just won't work. We have no >>>>> way of knowing if the 'fd' changed, but we can detect if it was closed >>>>> and then we'll end up just EBADF'ing the requests. >>>>> >>>>> So right now the answer is that we can support this just fine with >>>>> SQPOLL, but you better not dup and close the original fd. Which is not >>>>> ideal, but better than NOT being able to support it. >>>>> >>>>> Only other option I see is to to provide an io_uring_register() >>>>> command to update the fd/file associated with it. Which may be useful, >>>>> it allows a process to indeed to this, if it absolutely has to. >>>> >>>> Let's put aside such dirty hacks, at least until someone actually >>>> needs it. Ideally, for many reasons I'd prefer to get rid of >>> >>> BUt it is actually needed, otherwise we're even more in a limbo state of >>> "SQPOLL works for most things now, just not all". And this isn't that >>> hard to make right - on the flush() side, we just need to park/stall the >> >> I understand that it isn't hard, but I just don't want to expose it to >> the userspace, a) because it's a userspace API, so couldn't probably be >> killed in the future, b) works around kernel's problems, and so >> shouldn't really be exposed to the userspace in normal circumstances. >> >> And it's not generic enough because of a possible "many fds -> single >> file" mapping, and there will be a lot of questions and problems. >> >> e.g. if a process shares a io_uring with another process, then >> dup()+close() would require not only this hook but also additional >> inter-process synchronisation. And so on. > > I think you're blowing this out of proportion. Just to restate the I just think that if there is a potentially cleaner solution without involving userspace, we should try to look for it first, even if it would take more time. That was the point. > goal, but it's to have SQPOLL be as useful as the other modes. One of > those things is making non-registered files work. For some use cases, > registered files is fine, for others it's basically a non-starter.> With that out of the way, the included patch handles the "close ring > fd case". You're talking about the dup or receive case, or anything > that doesn't close an existing ring. And yes, that won't work as-is, > because we know have multiple fds for that particular ring. That boils > the case down to "we're now using this fd for the ring", and the only > requirement here would be to ensure you do a io_uring_enter() if you > decide to swap fds or use a new fd. Only caveat here is that we can't > make it automatic like we can for the "old fd gets closed" case, so > the app would absolutely have to ensure it enters the kernel if it > uses a new fd. > > Not really a huge deal imho in terms of API, especially since this > is into the realm of "nobody probably ever does this, or if they do, > then this requirement isn't really a problem". > >>> thread and clear the ring_fd/ring_file, then mark the ring as needing a >>> queue enter. On the queue enter, we re-set the fd/file if they're NULL, >>> unpark/unstall the thread. That's it. I'll write this up and test it. >>> >>>> fcheck(ctx->ring_fd) in favour of synchronisation in io_grab_files(), >>>> but I wish I knew how. >>> >>> That'd be nice, and apply equally to all cases as the SQPOLL case isn't >>> special at all anymore. >> >> I miss the whole story, have you asked fs guys about the problem? >> Or is it known that nothing would work? > > I haven't looked into it. Any chance you have someone in mind who can take a look? I don't think I have a chance to get to anyone in fs. -- Pavel Begunkov