Re: [PATCH 1/1] io_uring: fix leaks on IOPOLL and CQE_SKIP

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

On 4/16/22 00:51, Jens Axboe wrote:
> On 4/15/22 4:53 PM, Jens Axboe wrote:
>> On 4/15/22 4:41 PM, Pavel Begunkov wrote:
>>> On 4/15/22 23:03, Jens Axboe wrote:
>>>> On 4/15/22 3:05 PM, Pavel Begunkov wrote:
>>>>> On 4/12/22 17:46, Jens Axboe wrote:
>>>>>> On 4/12/22 10:41 AM, Jens Axboe wrote:
>>>>>>> On 4/12/22 10:24 AM, Pavel Begunkov wrote:
>>>>>>>> If all completed requests in io_do_iopoll() were marked with
>>>>>>>> REQ_F_CQE_SKIP, we'll not only skip CQE posting but also
>>>>>>>> io_free_batch_list() leaking memory and resources.
>>>>>>>>
>>>>>>>> Move @nr_events increment before REQ_F_CQE_SKIP check. We'll potentially
>>>>>>>> return the value greater than the real one, but iopolling will deal with
>>>>>>>> it and the userspace will re-iopoll if needed. In anyway, I don't think
>>>>>>>> there are many use cases for REQ_F_CQE_SKIP + IOPOLL.
>>>>>>>
>>>>>>> Ah good catch - yes probably not much practical concern, as the lack of
>>>>>>> ordering for file IO means that CQE_SKIP isn't really useful for that
>>>>>>> scenario.
>>>>>>
>>>>>> One potential snag is with the change we're now doing
>>>>>> io_cqring_ev_posted_iopoll() even if didn't post an event. Again
>>>>>> probably not a practical concern, but it is theoretically a violation
>>>>>> if an eventfd is used.
>>>>> Looks this didn't get applied. Are you concerned about eventfd?
>>>>
>>>> Yep, was hoping to get a reply back, so just deferred it for now.
>>>>
>>>>> Is there any good reason why the userspace can't tolerate spurious
>>>>> eventfd events? Because I don't think we should care this case
>>>>
>>>> I always forget the details on that, but we've had cases like this in
>>>> the past where some applications assume that if they got N eventfd
>>>> events, then are are also N events in the ring. Which granted is a bit
>>>> odd, but it does also make some sense. Why would you have more eventfd
>>>> events posted than events?
>>>
>>> For the same reason why it can get less eventfd events than there are
>>> CQEs, as for me it's only a communication channel but not a
>>> replacement for completion events.
>>
>> That part is inherently racy in that we might get some CQEs while we
>> respond to the initial eventfd notifications. But I'm totally agreeing
>> with you, and it doesn't seem like a big deal to me.
>>
>>> Ok, we don't want to break old applications, but it's a new most
>>> probably not widely used feature, and we can say that the userspace
>>> has to handle spurious eventfd.
>>
>> If I were to guess, I'd say it's probably epoll + eventfd conversions.
>> But it should just be made explicit. Since events reaped and checked
>> happen differently anyway, it seems like a bad assumption to make that
>> eventfd notifications == events available.
> 
> The patch is against the 5.19 branch, but it might be a better idea
> to do this for 5.18 as the 5.17 backport will then not need
> assistance. Can you send it against io_uring-5.18?

sure

-- 
Pavel Begunkov