Re: [RFC] io_uring: let IORING_OP_FILES_UPDATE support to choose fixed file slot

[Jens Axboe <axboe@kernel.dk>]

On 5/26/22 6:38 AM, Xiaoguang Wang wrote:
> One big issue with file registration feature is that it needs user
> space apps to maintain free slot info about io_uring's fixed file
> table, which really is a burden for development. Now since io_uring
> starts to choose free file slot for user space apps by using
> IORING_FILE_INDEX_ALLOC flag in accept or open operations, but they
> need app to uses direct accept or direct open, which as far as I know,
> some apps are not prepared to use direct accept or open yet.
> 
> To support apps, who still need real fds, use registration feature
> easier, let IORING_OP_FILES_UPDATE support to choose fixed file slot,
> which will return free file slot in cqe->res.

This looks good. In retrospect, the direct open/accept/etc really
should've just returned the direct descriptor in cqe->res, so we would
not have this odd "0 for success when you pick a slot, <slot> in
cqe->res if io_uring picks it". But this is consistent with the alloc
case.

Do you have a liburing test case too? I think we should just get this
done for 5.19 rather than spread it over multiple releases, since 5.19
introduced the alloc as well.

-- 
Jens Axboe