Re: [PATCH] io_uring: add io_uring_enter(2) fixed file support

[Pavel Begunkov <asml.silence@gmail.com>]

On 3/3/22 16:31, Jens Axboe wrote:
> On 3/3/22 7:40 AM, Jens Axboe wrote:
>> On 3/3/22 7:36 AM, Jens Axboe wrote:
>>> The only potential oddity here is that the fd passed back is not a
>>> legitimate fd. io_uring does support poll(2) on its file descriptor, so
>>> that could cause some confusion even if I don't think anyone actually
>>> does poll(2) on io_uring.
>>
>> Side note - the only implication here is that we then likely can't make
>> the optimized behavior the default, it has to be an IORING_SETUP_REG
>> flag which tells us that the application is aware of this limitation.
>> Though I guess close(2) might mess with that too... Hmm.
> 
> Not sure I can find a good approach for that. Tried out your patch and
> made some fixes:
> 
> - Missing free on final tctx free
> - Rename registered_files to registered_rings
> - Fix off-by-ones in checking max registration count
> - Use kcalloc
> - Rename ENTER_FIXED_FILE -> ENTER_REGISTERED_RING
> - Don't pass in tctx to io_uring_unreg_ringfd()
> - Get rid of forward declaration for adding tctx node
> - Get rid of extra file pointer in io_uring_enter()
> - Fix deadlock in io_ringfd_register()
> - Use io_uring_rsrc_update rather than add a new struct type
> 
> Patch I ran below.
> 
> Ran some testing here, and on my laptop, running:
> 
> axboe@m1pro-kvm ~/g/fio (master)> t/io_uring -N1 -s1 -f0
> polled=1, fixedbufs=1/0, register_files=1, buffered=0, QD=128
> Engine=io_uring, sq_ring=128, cq_ring=128
> submitter=0, tid=673
> IOPS=6627K, IOS/call=1/1, inflight=()
> IOPS=6995K, IOS/call=1/1, inflight=()
> IOPS=6992K, IOS/call=1/1, inflight=()
> IOPS=7005K, IOS/call=1/1, inflight=()
> IOPS=6999K, IOS/call=1/1, inflight=()
> 
> and with registered ring
> 
> axboe@m1pro-kvm ~/g/fio (master)> t/io_uring -N1 -s1 -f1
> polled=1, fixedbufs=1/0, register_files=1, buffered=0, QD=128
> Engine=io_uring, sq_ring=128, cq_ring=128
> submitter=0, tid=687
> ring register 0
> IOPS=7714K, IOS/call=1/1, inflight=()
> IOPS=8030K, IOS/call=1/1, inflight=()
> IOPS=8025K, IOS/call=1/1, inflight=()
> IOPS=8015K, IOS/call=1/1, inflight=()
> IOPS=8037K, IOS/call=1/1, inflight=()
> 
> which is about a 15% improvement, pretty massive...
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index ad3e0b0ab3b9..8a1f97054b71 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
[...]
>   static void *io_uring_validate_mmap_request(struct file *file,
>   					    loff_t pgoff, size_t sz)
>   {
> @@ -10191,12 +10266,23 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
>   	io_run_task_work();
>   
>   	if (unlikely(flags & ~(IORING_ENTER_GETEVENTS | IORING_ENTER_SQ_WAKEUP |
> -			       IORING_ENTER_SQ_WAIT | IORING_ENTER_EXT_ARG)))
> +			       IORING_ENTER_SQ_WAIT | IORING_ENTER_EXT_ARG |
> +			       IORING_ENTER_REGISTERED_RING)))
>   		return -EINVAL;
>   
> -	f = fdget(fd);
> -	if (unlikely(!f.file))
> -		return -EBADF;
> +	if (flags & IORING_ENTER_REGISTERED_RING) {
> +		struct io_uring_task *tctx = current->io_uring;
> +
> +		if (fd >= IO_RINGFD_REG_MAX || !tctx)
> +			return -EINVAL;
> +		f.file = tctx->registered_rings[fd];

btw, array_index_nospec(), possibly not only here.

-- 
Pavel Begunkov