Re: [PATCH 3/3] io_uring: refactor sendmsg/recvmsg iov managing

[Jens Axboe <axboe@kernel.dk>]

On 2/5/21 3:06 AM, Pavel Begunkov wrote:
> On 05/02/2021 09:58, Stefan Metzmacher wrote:
>> Hi Pavel,
>>
>>>>>  static int io_sendmsg_copy_hdr(struct io_kiocb *req,
>>>>>  			       struct io_async_msghdr *iomsg)
>>>>>  {
>>>>> -	iomsg->iov = iomsg->fast_iov;
>>>>>  	iomsg->msg.msg_name = &iomsg->addr;
>>>>> +	iomsg->free_iov = iomsg->fast_iov;
>>>>
>>>> Why this? Isn't the idea of this patch that free_iov is never == fast_iov?
>>>
>>> That's a part of __import_iovec() and sendmsg_copy_msghdr() API, you pass
>>> fast_iov as such and get back NULL or a newly allocated one in it.
>> I think that should at least get a comment to make this clear and
>> maybe a temporary variable like this:
>>
>> tmp_iov = iomsg->fast_iov;
>> __import_iovec(..., &tmp_iov, ...);
>> iomsg->free_iov = tmp_iov;
> 
> I'd rather disagree. It's a well known (ish) API, and I deliberately
> placed such assignments right before import_iovec/etc.

Agree on that, it's kind of a weird idiom, but it's used throughout the
kernel. However:

>>>> kfree() handles NULL, or is this a hot path and we want to avoid a function call?
>>>
>>> Yes, the hot path here is not having iov allocated, and Jens told before
>>> that he had observed overhead for a similar place in io_[read,write].
>>
>> Ok, a comment would also help here...

I do agree on that one, since otherwise we get patches for it as has
been proven by the few other spots... I'll add then when queueing this
up.

-- 
Jens Axboe