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

[Jens Axboe <axboe@kernel.dk>]

On 3/3/22 6:52 PM, Pavel Begunkov wrote:
> 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.

Yeah, was thinking that earlier too in fact but forgot about it. Might
as well, though I don't think it's strictly required as it isn't a user
table.

-- 
Jens Axboe