Re: [PATCH v3 1/2] io_uring: change the poll events to be 32-bits

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

On 16/06/2020 21:45, Jens Axboe wrote:
> On 6/16/20 7:58 AM, Jens Axboe wrote:
>> On 6/15/20 9:04 PM, Jiufei Xue wrote:
>>>
>>>
>>> On 2020/6/15 下午11:09, Jens Axboe wrote:
>>>> On 6/14/20 8:49 PM, Jiufei Xue wrote:
>>>>> Hi Jens,
>>>>>
>>>>> On 2020/6/13 上午12:48, Jens Axboe wrote:
>>>>>> On 6/12/20 8:58 AM, Jens Axboe wrote:
>>>>>>> On 6/11/20 8:30 PM, Jiufei Xue wrote:
>>>>>>>> poll events should be 32-bits to cover EPOLLEXCLUSIVE.
>>>>>>>>
>>>>>>>> Signed-off-by: Jiufei Xue <jiufei.xue@linux.alibaba.com>
>>>>>>>> ---
>>>>>>>>  fs/io_uring.c                 | 4 ++--
>>>>>>>>  include/uapi/linux/io_uring.h | 2 +-
>>>>>>>>  tools/io_uring/liburing.h     | 2 +-
>>>>>>>>  3 files changed, 4 insertions(+), 4 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>>>>>> index 47790a2..6250227 100644
>>>>>>>> --- a/fs/io_uring.c
>>>>>>>> +++ b/fs/io_uring.c
>>>>>>>> @@ -4602,7 +4602,7 @@ static void io_poll_queue_proc(struct file *file, struct wait_queue_head *head,
>>>>>>>>  static int io_poll_add_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>>>>>>>  {
>>>>>>>>  	struct io_poll_iocb *poll = &req->poll;
>>>>>>>> -	u16 events;
>>>>>>>> +	u32 events;
>>>>>>>>  
>>>>>>>>  	if (unlikely(req->ctx->flags & IORING_SETUP_IOPOLL))
>>>>>>>>  		return -EINVAL;
>>>>>>>> @@ -8196,7 +8196,7 @@ static int __init io_uring_init(void)
>>>>>>>>  	BUILD_BUG_SQE_ELEM(28, /* compat */   int, rw_flags);
>>>>>>>>  	BUILD_BUG_SQE_ELEM(28, /* compat */ __u32, rw_flags);
>>>>>>>>  	BUILD_BUG_SQE_ELEM(28, __u32,  fsync_flags);
>>>>>>>> -	BUILD_BUG_SQE_ELEM(28, __u16,  poll_events);
>>>>>>>> +	BUILD_BUG_SQE_ELEM(28, __u32,  poll_events);
>>>>>>>>  	BUILD_BUG_SQE_ELEM(28, __u32,  sync_range_flags);
>>>>>>>>  	BUILD_BUG_SQE_ELEM(28, __u32,  msg_flags);
>>>>>>>>  	BUILD_BUG_SQE_ELEM(28, __u32,  timeout_flags);
>>>>>>>> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
>>>>>>>> index 92c2269..afc7edd 100644
>>>>>>>> --- a/include/uapi/linux/io_uring.h
>>>>>>>> +++ b/include/uapi/linux/io_uring.h
>>>>>>>> @@ -31,7 +31,7 @@ struct io_uring_sqe {
>>>>>>>>  	union {
>>>>>>>>  		__kernel_rwf_t	rw_flags;
>>>>>>>>  		__u32		fsync_flags;
>>>>>>>> -		__u16		poll_events;
>>>>>>>> +		__u32		poll_events;
>>>>>>>>  		__u32		sync_range_flags;
>>>>>>>>  		__u32		msg_flags;
>>>>>>>>  		__u32		timeout_flags;
>>>>>>>
>>>>>>> We obviously have the space in there as most other flag members are 32-bits, but
>>>>>>> I'd want to double check if we're not changing the ABI here. Is this always
>>>>>>> going to be safe, on any platform, regardless of endianess etc?
>>>>>>
>>>>>> Double checked, and as I feared, we can't safely do this. We'll have to
>>>>>> do something like the below, grabbing an unused bit of the poll mask
>>>>>> space and if that's set, then store the fact that EPOLLEXCLUSIVE is set.
>>>>>> So probably best to turn this just into one patch, since it doesn't make
>>>>>> a lot of sense to do it as a prep patch at that point.
>>>>>>
>>>>> Yes, Agree about that. But I also fear that if the unused bit is used
>>>>> in the feature, it will bring unexpected behavior.
>>>>
>>>> Yeah, it's certainly not the prettiest and could potentially be fragile.
>>>> I'm open to suggestions, we need some way of signaling that the 32-bit
>>>> variant of the poll_events should be used. We could potentially make
>>>> this work by doing explicit layout for big endian vs little endian, that
>>>> might be prettier and wouldn't suffer from the "grab some random bit"
>>>> issue.
>>>>
>>> Thank you for your suggestion, I will think about it.
>>>
>>>>>> This does have the benefit of not growing io_poll_iocb. With your patch,
>>>>>> it'd go beyond a cacheline, and hence bump the size of the entire
>>>>>> io_iocb as well, which would be very unfortunate.
>>>>>>
>>>>> events in io_poll_iocb is 32-bits already, so why it will bump the
>>>>> size of the io_iocb structure with my patch? 
>>>>
>>>> It's not 32-bits already, it's a __poll_t type which is 16-bits only.
>>>>
>>> Yes, it is a __poll_t type, but I found that __poll_t type is 32-bits
>>> with the definition below:
>>>
>>> typedef unsigned __bitwise __poll_t;
>>>
>>> And I also investigate it with crash:
>>> crash> io_poll_iocb -ox
>>> struct io_poll_iocb {
>>>    [0x0] struct file *file;
>>>          union {
>>>    [0x8]     struct wait_queue_head *head;
>>>    [0x8]     u64 addr;
>>>          };
>>>   [0x10] __poll_t events;
>>>   [0x14] bool done;
>>>   [0x15] bool canceled;
>>>   [0x18] struct wait_queue_entry wait;
>>> }
>>
>> Yeah you're right, not sure why I figured it was 16-bits. But just
>> checking on my default build:
>>
>> axboe@x1 ~/gi/linux-block (block-5.8)> pahole -C io_poll_iocb fs/io_uring.o
>> struct io_poll_iocb {
>> 	struct file *              file;                 /*     0     8 */
>> 	union {
>> 		struct wait_queue_head * head;           /*     8     8 */
>> 		u64                addr;                 /*     8     8 */
>> 	};                                               /*     8     8 */
>> 	__poll_t                   events;               /*    16     4 */
>> 	bool                       done;                 /*    20     1 */
>> 	bool                       canceled;             /*    21     1 */
>>
>> 	/* XXX 2 bytes hole, try to pack */
>>
>> 	struct wait_queue_entry wait;                    /*    24    40 */
>>
>> 	/* size: 64, cachelines: 1, members: 6 */
>> 	/* sum members: 62, holes: 1, sum holes: 2 */
>> };
>>
>> and it's definitely 64-bytes in total size (as it should be), and the
>> 'events' is indeed 32-bits as well.
>>
>> So at least that's good, we don't need anything extra in there. If we
>> can solve the endian issue, then it should be trivial to use the full
>> 32-bits for the flags in the sqe.
> 
> To get back to something that can move us forward, something like the
> below I _think_ will work. Then applications just use poll32_events and
> we don't have to handle this in any special way. Care to base on that
> and re-send the change?
> 
> 
> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> index 92c22699a5a7..5b3f6bd59437 100644
> --- a/include/uapi/linux/io_uring.h
> +++ b/include/uapi/linux/io_uring.h
> @@ -31,7 +31,16 @@ struct io_uring_sqe {
>  	union {
>  		__kernel_rwf_t	rw_flags;
>  		__u32		fsync_flags;
> -		__u16		poll_events;
> +		struct {
> +#ifdef __BIG_ENDIAN_BITFIELD
> +			__u16	__unused_poll;
> +			__u16	poll_events;
> +#else
> +			__u16	poll_events;
> +			__u16	__unused_poll;
> +#endif
> +		};
> +		__u32		poll32_events;
>  		__u32		sync_range_flags;
>  		__u32		msg_flags;
>  		__u32		timeout_flags;
> 

That changes layout for big endian, isn't it? It's not ABI compatible.

-- 
Pavel Begunkov