Re: [RFC v2 02/19] skbuff: pass a struct ubuf_info in msghdr

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

On 1/12/22 03:39, Hao Xu wrote:
> 在 2022/1/11 下午11:50, Pavel Begunkov 写道:
>> On 1/11/22 13:51, Hao Xu wrote:
>>> 在 2021/12/21 下午11:35, Pavel Begunkov 写道:
>>>> Instead of the net stack managing ubuf_info, allow to pass it in from
>>>> outside in a struct msghdr (in-kernel structure), so io_uring can make
>>>> use of it.
>>>>
>>>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>>>> ---
>>> Hi Pavel,
>>> I've some confusions here since I have a lack of
>>> network knowledge.
>>> The first one is why do we make ubuf_info visible
>>> for io_uring. Why not just follow the old MSG_ZEROCOPY
>>> logic?
>>
>> I assume you mean leaving allocation up and so in socket awhile the
>> patchset let's io_uring to manage and control ubufs. In short,
>> performance and out convenience
>>
>> TL;DR;
>> First, we want a nice and uniform API with io_uring, i.e. posting
>> an CQE instead of polling an err queue/etc., and for that the network
>> will need to know about io_uring ctx in some way. As an alternative it
>> may theoretically be registered in socket, but it'll quickly turn into
>> a huge mess, consider that it's a many to many relation b/w io_uring and
>> sockets. The fact that io_uring holds refs to files will only complicate
>> it.
> Make sense to me, thanks.
>>
>> It will also limit API. For instance, we won't be able to use a single
>> ubuf with several different sockets.
> Is there any use cases for this multiple sockets with single
> notification?

Don't know, scatter send maybe? It's just one of those moments when
a design that feels right (to me) yields more flexibility than was
initially planned, which is definitely a good thing


>> Another problem is performance, registration or some other tricks
>> would some additional sync. It'd also need sync on use, say it's
>> just one rcu_read, but the problem that it only adds up to complexity
>> and prevents some other optimisations. E.g. we amortise to ~0 atomics
>> getting refs on skb setups based on guarantees io_uring provides, and
>> not only. SKBFL_MANAGED_FRAGS can only work with pages being controlled
>> by the issuer, and so it needs some context as currently provided by
>> ubuf. io_uring also caches ubufs, which relies on io_uring locking, so
>> it removes kmalloc/free for almost zero overhead.
>>
>>
>>> The second one, my understanding about the buffer
>>> lifecycle is that the kernel side informs
>>> the userspace by a cqe generated by the ubuf_info
>>> callback that all the buffers attaching to the
>>> same notifier is now free to use when all the data
>>> is sent, then why is the flush in 13/19 needed as
>>> it is at the submission period?
>>
>> Probably I wasn't clear enough. A user has to flush a notifier, only
>> then it's expected to post an CQE after all buffers attached to it
>> are freed. io_uring holds one ubuf ref, which will be release on flush.
> I see, I saw another ref inc in skb_zcopy_set() which I previously
> misunderstood and thus thought there was only one refcount. Thanks!
>> I also need to add a way to flush without send.
>>
>> Will spend some time documenting for next iteration.

-- 
Pavel Begunkov