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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: <4c26deb4-65c6-57c0-2b51-02861ef8558c@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/07/2020 21:50, Pavel Begunkov wrote: > On 09/07/2020 21:36, Kanchan Joshi wrote: >> On Thu, Jul 9, 2020 at 7:36 PM Jens Axboe wrote: >>> >>> On 7/9/20 8:00 AM, Christoph Hellwig wrote: >>>> On Thu, Jul 09, 2020 at 07:58:04AM -0600, Jens Axboe wrote: >>>>>> We don't actually need any new field at all. By the time the write >>>>>> returned ki_pos contains the offset after the write, and the res >>>>>> argument to ->ki_complete contains the amount of bytes written, which >>>>>> allow us to trivially derive the starting position. >> >> Deriving starting position was not the purpose at all. >> But yes, append-offset is not needed, for a different reason. >> It was kept for uring specific handling. Completion-result from lower >> layer was always coming to uring in ret2 via ki_complete(....,ret2). >> And ret2 goes to CQE (and user-space) without any conversion in between. >> For polled-completion, there is a short window when we get ret2 but cannot >> write into CQE immediately, so thought of storing that in append_offset >> (but should not have done, solving was possible without it). > > fwiw, there are more cases when it's postponed. > >> FWIW, if we move to indirect-offset approach, append_offset gets >> eliminated automatically, because there is no need to write to CQE >> itself. > > Right, for the indirect approach we can write offset right after getting it. Take it back, as you mentioned with task_work, we may need the right context. BTW, there is one more way to get space for it, and it would also shed 8 bytes from io_kiocb, but that would need some work to be done. > If not, then it's somehow stored in an CQE, so may be placed into > existing req->{result,cflags}, which mimic CQE's fields. > >> >>>>> Then let's just do that instead of jumping through hoops either >>>>> justifying growing io_rw/io_kiocb or turning kiocb into a global >>>>> completion thing. >>>> >>>> Unfortunately that is a totally separate issue - the in-kernel offset >>>> can be trivially calculated. But we still need to figure out a way to >>>> pass it on to userspace. The current patchset does that by abusing >>>> the flags, which doesn't really work as the flags are way too small. >>>> So we somewhere need to have an address to do the put_user to. >>> >>> Right, we're just trading the 'append_offset' for a 'copy_offset_here' >>> pointer, which are stored in the same spot... >> >> The address needs to be stored somewhere. And there does not seem >> other option but to use io_kiocb? >> The bigger problem with address/indirect-offset is to be able to write to it >> during completion as process-context is different. Will that require entering >> into task_work_add() world, and may make it costly affair? >> >> Using flags have not been liked here, but given the upheaval involved so >> far I have begun to feel - it was keeping things simple. Should it be >> reconsidered? >> >> >> -- >> Joshi >> > -- Pavel Begunkov