Re: [PATCH] io_uring: add support for IORING_REGISTER_FILES_UPDATE

[Jeff Moyer <jmoyer@redhat.com>]

Jens Axboe <axboe@kernel.dk> 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?

>> Can you post the man page update along with this?
>
> Yes, I'll write the documentation too, just wanted consensus on the
> approach before I wrote up documentation.

Gotcha.

-Jeff