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

[Hao Xu <haoxu@linux.alibaba.com>]

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?
> 
> 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.
>