On 10/4/19 10:17 AM, Jeff Moyer wrote: > Jens Axboe <axboe@xxxxxxxxx> writes: > >> On 10/4/19 10:03 AM, Jeff Moyer wrote: >>> Jens Axboe <axboe@xxxxxxxxx> writes: >>> >>>> On 10/4/19 9:34 AM, Jeff Moyer wrote: >>>>> If I'm reading this (and the code) right, that means you can't add files >>>>> to a set. Wouldn't that be a useful thing to do, instead of just >>>>> replacing existing ones? >>>> >>>> You can add files to a set, you just can't grow a set beyond the size >>>> you originally registered. I actually forgot to post the pre-patch for >>>> this, which is: >>>> >>>> http://git.kernel.dk/cgit/linux-block/commit/?h=for-5.5/io_uring&id=fb3e60f87aa43f4f047f01243d6be54dadd9d67a >>>> >>>> This allows registering -1 as the fd, so you could register 10 files, >>>> but an array of size 500 (for example), and the last 490 fds are just >>>> -1. Then you can use the IORING_REGISTER_FILES_UPDATE to replace those >>>> empty fds with real files later on. >>> >>> That makes more sense, but still requires a priori knowledge of how many >>> files you'll need to work with, otherwise you're back to >>> unregister/register dance. Is it really that hard to grow on demand? >> >> It's not, it's just more efficient to add a file through replace, than it >> is to alloc a new array, copy things over, free it. That also impacts the >> application side of things, since that has to maintain an array of >> descriptors so that it knows what fd maps to what index. > > Yeah, that's a good point about the application side. > >> If you expose growing, you also have some kind of obligation to make it >> efficient, and I just don't think that's possible. But there's nothing >> preventing this API from supporting it, you'd just do an update with >> offset == current_array_size, and then nr_args as what to grow with. >> Currently that would -EINVAL, but it could be added as a feature. > > OK, I'm fine with the API as-is. If someone asks, growing can be added > later. Sounds good, thanks. I'll send out a v2 with the prep patch included. -- Jens Axboe
