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Fri, 12 Jun 2020 11:34:52 -0700 (PDT) To: Jens Axboe , io-uring References: <12b44e81-332e-e53c-b5fa-09b7bf9cc082@gmail.com> <6f6e1aa2-87f6-b853-5009-bf0961065036@kernel.dk> <5347123a-a0d5-62cf-acdf-6b64083bdc74@gmail.com> <868c9ef4-ab31-8c63-cace-9fd99c58cbb2@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: [RFC] do_iopoll() and *grab_env() Message-ID: <3688a25e-c405-309f-cc87-96596a5d0ed2@gmail.com> Date: Fri, 12 Jun 2020 21:33:28 +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: <868c9ef4-ab31-8c63-cace-9fd99c58cbb2@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 12/06/2020 21:02, Jens Axboe wrote: > On 6/12/20 11:55 AM, Jens Axboe wrote: >> On 6/12/20 11:30 AM, Pavel Begunkov wrote: >>> On 12/06/2020 20:02, Jens Axboe wrote: >>>> On 6/11/20 9:54 AM, Pavel Begunkov wrote: >>>>> io_do_iopoll() can async punt a request with io_queue_async_work(), >>>>> so doing io_req_work_grab_env(). The problem is that iopoll() can >>>>> be called from who knows what context, e.g. from a completely >>>>> different process with its own memory space, creds, etc. >>>>> >>>>> io_do_iopoll() { >>>>> ret = req->poll(); >>>>> if (ret == -EAGAIN) >>>>> io_queue_async_work() >>>>> ... >>>>> } >>>>> >>>>> >>>>> I can't find it handled in io_uring. Can this even happen? >>>>> Wouldn't it be better to complete them with -EAGAIN? >>>> >>>> I don't think a plain -EAGAIN complete would be very useful, it's kind >>>> of a shitty thing to pass back to userspace when it can be avoided. For >>>> polled IO, we know we're doing O_DIRECT, or using fixed buffers. For the >>>> latter, there's no problem in retrying, regardless of context. For the >>>> former, I think we'd get -EFAULT mapping the IO at that point, which is >>>> probably reasonable. I'd need to double check, though. >>> >>> It's shitty, but -EFAULT is the best outcome. I care more about not >>> corrupting another process' memory if addresses coincide. AFAIK it can >>> happen because io_{read,write} will use iovecs for punted re-submission. >>> >>> >>> Unconditional in advance async_prep() is too heavy to be good. I'd love to >>> see something more clever, but with -EAGAIN users at least can handle it. >> >> So how about we just grab ->task for the initial issue, and retry if we >> find it through -EAGAIN and ->task == current. That'll be the most >> common case, by far, and it'll prevent passes back -EAGAIN when we >> really don't have to. If the task is different, then -EAGAIN makes more >> sense, because at that point we're passing back -EAGAIN because we >> really cannot feasibly handle it rather than just as a convenience. Yeah, I was even thinking to drag it through task_work just to call *grab_env() there. Looks reasonable to me. > Something like this, totally untested. And wants a comment too. Looks like it. Would you leave this to me? There is another issue with cancellation requiring ->task, It'd be easier to keep them together. > > diff --git a/fs/io_uring.c b/fs/io_uring.c > index 155f3d830ddb..15806f71b33e 100644 > --- a/fs/io_uring.c > +++ b/fs/io_uring.c > @@ -1727,6 +1728,12 @@ static int io_put_kbuf(struct io_kiocb *req) > return cflags; > } > > +static inline void req_set_fail_links(struct io_kiocb *req) > +{ > + if ((req->flags & (REQ_F_LINK | REQ_F_HARDLINK)) == REQ_F_LINK) > + req->flags |= REQ_F_FAIL_LINK; > +} > + > /* > * Find and free completed poll iocbs > */ > @@ -1767,8 +1774,14 @@ static void io_iopoll_queue(struct list_head *again) > do { > req = list_first_entry(again, struct io_kiocb, list); > list_del(&req->list); > - refcount_inc(&req->refs); > - io_queue_async_work(req); > + if (req->task == current) { > + refcount_inc(&req->refs); > + io_queue_async_work(req); > + } else { > + io_cqring_add_event(req, -EAGAIN); > + req_set_fail_links(req); > + io_put_req(req); > + } > } while (!list_empty(again)); > } > > @@ -1937,12 +1950,6 @@ static void kiocb_end_write(struct io_kiocb *req) > file_end_write(req->file); > } > > -static inline void req_set_fail_links(struct io_kiocb *req) > -{ > - if ((req->flags & (REQ_F_LINK | REQ_F_HARDLINK)) == REQ_F_LINK) > - req->flags |= REQ_F_FAIL_LINK; > -} > - > static void io_complete_rw_common(struct kiocb *kiocb, long res) > { > struct io_kiocb *req = container_of(kiocb, struct io_kiocb, rw.kiocb); > @@ -2137,6 +2144,8 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe, > > kiocb->ki_flags |= IOCB_HIPRI; > kiocb->ki_complete = io_complete_rw_iopoll; > + req->task = current; > + get_task_struct(current); > req->result = 0; > req->iopoll_completed = 0; > } else { > -- Pavel Begunkov