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Fri, 11 Sep 2020 12:25:28 -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> <33a6730c-8e0c-e34f-9094-c256a13961cd@gmail.com> <163d7844-e2a4-2739-af4e-79f4a3ec9a1d@kernel.dk> <73b8038a-eedf-04f7-6991-938512faaee6@kernel.dk> 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: <8f953abe-cdcd-18a9-fe06-2c0480cddff5@gmail.com> Date: Fri, 11 Sep 2020 22:23:02 +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: <73b8038a-eedf-04f7-6991-938512faaee6@kernel.dk> 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 10/09/2020 21:18, Jens Axboe wrote: > On 9/10/20 7:11 AM, Jens Axboe wrote: >> On 9/10/20 6:37 AM, Pavel Begunkov wrote: >>> 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. >> >> Regardless of whether or not we can eliminate that need, at least it'll >> be a relaxing of the restriction, not an increase of it. It'll never >> hurt to do an extra system call for the case where you're swapping fds. >> I do get your point, I just don't think it's a big deal. > > BTW, I don't see how we can ever get rid of a need to enter the kernel, > we'd need some chance at grabbing the updated ->files, for instance. Thanks for taking a look. Yeah, agree, it should get it from somewhere, and that reminds me that we have a similar situation with sqo_mm -- it grabs it from the task-creator and keeps it to the end... Do we really need to set ->files of another thread? Retaining to the end seem to work well enough with mm. And we need, then it would be more consistent to replace mm there as well. > Might be possible to hold a reference to the task and grab it from > there, though feels a bit iffy to hold a task reference from the ring on > the task that holds a reference to the ring. Haven't looked too close, > should work though as this won't hold a file/files reference, it's just > a freeing reference. BTW, if the process-creator dies, then its ->files might be killed and ->sqo_files become dangling, so should be invalidated. Your approach with a task's reference probably handles it naturally. -- Pavel Begunkov