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Tue, 16 Jun 2020 11:45:22 -0700 (PDT) Subject: Re: [PATCH v3 1/2] io_uring: change the poll events to be 32-bits From: Jens Axboe To: Jiufei Xue , io-uring@vger.kernel.org Cc: joseph.qi@linux.alibaba.com References: <1591929018-73954-1-git-send-email-jiufei.xue@linux.alibaba.com> <1591929018-73954-2-git-send-email-jiufei.xue@linux.alibaba.com> <9e251ae9-ffe1-d9ea-feb5-cb9e641aeefb@kernel.dk> <7a311161-839c-3927-951d-3ce2bc7aa5d4@linux.alibaba.com> <967819fd-84c5-9329-60b6-899a2708849e@kernel.dk> Message-ID: <659bda5d-2da0-b092-9a66-1c4c4d89501a@kernel.dk> Date: Tue, 16 Jun 2020 12:45:20 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <967819fd-84c5-9329-60b6-899a2708849e@kernel.dk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org 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 >>>>>>> --- >>>>>>> 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