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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: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. 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