On 7/9/20 7:26 AM, Christoph Hellwig wrote: > On Thu, Jul 09, 2020 at 12:10:36PM +0100, Matthew Wilcox wrote: >> On Thu, Jul 09, 2020 at 11:17:05AM +0100, Christoph Hellwig wrote: >>> I really don't like this series at all. If saves a single pointer >>> but introduces a complicated machinery that just doesn't follow any >>> natural flow. And there doesn't seem to be any good reason for it to >>> start with. >> >> Jens doesn't want the kiocb to grow beyond a single cacheline, and we >> want the ability to set the loff_t in userspace for an appending write, >> so the plan was to replace the ki_complete member in kiocb with an >> loff_t __user *ki_posp. >> >> I don't think it's worth worrying about growing kiocb, personally, >> but this seemed like the easiest way to make room for a new pointer. > > The user offset pointer has absolutely no business in the the kiocb > itself - it is a io_uring concept which needs to go into the io_kiocb, Nobody disagrees on that. > which has 14 bytes left in the last cache line in my build. It would > fit in very well there right next to the result and user pointer. Per-op data should not spill into the io_kiocb itself. And I absolutely hate arguments like "oh there's still 14 bytes in there", because then there's 6, then there's none, and now we're going into the next cacheline. io_kiocb is already too fat, it should be getting slimmer, not bigger. And the append write stuff is not nearly interesting enough to a) grow io_kiocb, b) warrant a special case for op private data in the io_kiocb itself. -- Jens Axboe
