On 7/9/20 12:36 PM, Kanchan Joshi wrote: > On Thu, Jul 9, 2020 at 7:36 PM Jens Axboe <axboe@xxxxxxxxx> 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, if we move to indirect-offset approach, append_offset gets > eliminated automatically, because there is no need to write to CQE > itself. > >>>> 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? That is where it belongs, not sure this was ever questioned. And inside io_rw at that. > 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? It might, if you have IRQ context for the completion. task_work isn't expensive, however. It's not like a thread offload. > 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? It's definitely worth considering, especially since we can use cflags like Pavel suggested upfront and not need any extra storage. But it brings us back to the 32-bit vs 64-bit discussion, and then using blocks instead of bytes. Which isn't exactly super pretty. -- Jens Axboe
