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? 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
