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@xxxxxxxxxxxxxxxxx>
>>>>>>> ---
>>>>>>> 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;
--
Jens Axboe
