io_uring is zero copy or not?

io_uring is zero copy or not?

[Feng Li]

Hi Jens,

I'm interested in the io_uring, and have a doubt about it.
If I setup io_uring with IORING_REGISTER_BUFFERS,
the kernel doesn't need copy data from user space to the kernel, right?
Does it mean this is a zero-copy feature?

From the code, kernel directly get the userspace virtual address's
pages and store them:

2475 pret = get_user_pages_longterm(ubuf, nr_pages, FOLL_WRITE,
2476 pages, vmas);



-- 
Thanks and Best Regards,
Feng Li(Alex)

Re: io_uring is zero copy or not?

[Dongli Zhang]

On 4/15/19 11:09 AM, Feng Li wrote:
> Hi Jens,
> 
> I'm interested in the io_uring, and have a doubt about it.
> If I setup io_uring with IORING_REGISTER_BUFFERS,
> the kernel doesn't need copy data from user space to the kernel, right?
> Does it mean this is a zero-copy feature?

Doesn't it depends on the definition of 'copy'?

There is still a copy from original userspace pages to the registered buffer?

Dongli Zhang

> 
> From the code, kernel directly get the userspace virtual address's
> pages and store them:
> 
> 2475 pret = get_user_pages_longterm(ubuf, nr_pages, FOLL_WRITE,
> 2476 pages, vmas);
> 
> 
> 

Re: io_uring is zero copy or not?

[Feng Li]

Hi DongLi,

Thanks for your response.

The 'copy' means the data from userspace buffer could be direct to be
written to the device.
The kernel doesn't need to allocate a new buffer and copy the data in it.
Copy data is a wast for performance.

I don't know the block layer need to allocate a new buffer or not.
From the io_uring.c code, the kernel directly grab the pages of
userspace data buffer through the `get_user_pages`.

Dongli Zhang <dongli.zhang@oracle.com> 于2019年4月15日周一 上午11:21写道：
>
>
>
> On 4/15/19 11:09 AM, Feng Li wrote:
> > Hi Jens,
> >
> > I'm interested in the io_uring, and have a doubt about it.
> > If I setup io_uring with IORING_REGISTER_BUFFERS,
> > the kernel doesn't need copy data from user space to the kernel, right?
> > Does it mean this is a zero-copy feature?
>
> Doesn't it depends on the definition of 'copy'?
>
> There is still a copy from original userspace pages to the registered buffer?
>
> Dongli Zhang
>
> >
> > From the code, kernel directly get the userspace virtual address's
> > pages and store them:
> >
> > 2475 pret = get_user_pages_longterm(ubuf, nr_pages, FOLL_WRITE,
> > 2476 pages, vmas);
> >
> >
> >



-- 
Thanks and Best Regards,
Feng Li(Alex)