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

[Jens Axboe <axboe@kernel.dk>]

On 5/28/22 6:28 AM, Jens Axboe wrote:
> On 5/28/22 3:45 AM, Xiaoguang Wang wrote:
>> hi Hao,
>>
>>> Hi Xiaoguang,
>>>
>>> On 5/26/22 20:38, 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.
>>>>
>>>> TODO list:
>>>>      Need to prepare liburing corresponding helpers.
>>>>
>>>> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
>>>> ---
>>>>   fs/io_uring.c                 | 50 ++++++++++++++++++++++++++++++++++---------
>>>>   include/uapi/linux/io_uring.h |  1 +
>>>>   2 files changed, 41 insertions(+), 10 deletions(-)
>>>>
>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>> index 9f1c682d7caf..d77e6bbec81c 100644
>>>> --- a/fs/io_uring.c
>>>> +++ b/fs/io_uring.c
>>>> @@ -680,6 +680,7 @@ struct io_rsrc_update {
>>>>       u64                arg;
>>>>       u32                nr_args;
>>>>       u32                offset;
>>>> +    u32                flags;
>>>>   };
>>>>     struct io_fadvise {
>>>> @@ -7970,14 +7971,23 @@ static int io_async_cancel(struct io_kiocb *req, unsigned int issue_flags)
>>>>       return 0;
>>>>   }
>>>>   +#define IORING_FILES_UPDATE_INDEX_ALLOC 1
>>>> +
>>>>   static int io_rsrc_update_prep(struct io_kiocb *req,
>>>>                   const struct io_uring_sqe *sqe)
>>>>   {
>>>> +    u32 flags = READ_ONCE(sqe->files_update_flags);
>>>> +
>>>>       if (unlikely(req->flags & (REQ_F_FIXED_FILE | REQ_F_BUFFER_SELECT)))
>>>>           return -EINVAL;
>>>> -    if (sqe->rw_flags || sqe->splice_fd_in)
>>>> +    if (sqe->splice_fd_in)
>>>> +        return -EINVAL;
>>>> +    if (flags & ~IORING_FILES_UPDATE_INDEX_ALLOC)
>>>> +        return -EINVAL;
>>>> +    if ((flags & IORING_FILES_UPDATE_INDEX_ALLOC) && READ_ONCE(sqe->len) != 1)
>>>
>>> How about allowing multiple fd update in IORING_FILES_UPDATE_INDEX_ALLOC
>>> case? For example, using the sqe->addr(the fd array) to store the slots we allocated, and let cqe return the number of slots allocated.
>> Good idea, I'll try in patch v2, thanks.
>> Jens, any comments about this patch? At least It's really helpful to our
>> internal apps based on io_uring :)
> 
> I like this suggestion too, other thoughts in reply to the original.

BTW, if you have time, would be great to get this done for 5.19. It
makes the whole thing more consistent and makes it so that 5.19 has
(hopefully) all the alloc bits for direct descriptors.

-- 
Jens Axboe