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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe