Question about vmsplice + SPLICE_F_GIFT

Question about vmsplice + SPLICE_F_GIFT

[Leonardo Bras]

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Hello everybody,

I am not sure if this is the right place to be asking this. If is not,
I apologize for the inconvenience. Also, please tell me where is a
better way to as these questions.

I am trying to create a basic C code to test vmsplice + SPLICE_F_GIFT
for moving memory pages between two processes without copying.

I have followed the man pages and several recipes across the web, but I
could not reproduce it yet.

Basically, I am doing:
Sending process:
- malloc + memcpy for generating pages to transfer
- vmsplice with SPLICE_F_GIFT sending over named pipe (in a loop)
Receiving process:
- Create mmaped file to receive the pages
- splice with SPLICE_F_MOVE receiving from named pipe (in a loop)

I have seen the SPLICE_F_MOVE being used on steal ops from the
'pipebuffer', but I couldn't find a way to call it from splice.

Questions: 
It does what I think it does? (reassign memory pages from a process to
another)
If so, does page gifting still works?
If so, is there a basic recipe to test it's workings?

Thank you for reading,

Leonardo Brás 

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Re: Question about vmsplice + SPLICE_F_GIFT

[Leonardo Bras]

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On Tue, 2019-07-23 at 17:33 -0300, Leonardo Bras wrote:
> I have seen the SPLICE_F_MOVE being used on steal ops from the
> 'pipebuffer', but I couldn't find a way to call it from splice.

Sorry, typo here. I meant:

> I have seen the SPLICE_F_GIFT being used on steal ops [...]

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Re: Question about vmsplice + SPLICE_F_GIFT

[Miklos Szeredi]

On Tue, Jul 23, 2019 at 10:33 PM Leonardo Bras <leonardo@linux.ibm.com> wrote:
>
> Hello everybody,
>
> I am not sure if this is the right place to be asking this. If is not,
> I apologize for the inconvenience. Also, please tell me where is a
> better way to as these questions.
>
> I am trying to create a basic C code to test vmsplice + SPLICE_F_GIFT
> for moving memory pages between two processes without copying.
>
> I have followed the man pages and several recipes across the web, but I
> could not reproduce it yet.
>
> Basically, I am doing:
> Sending process:
> - malloc + memcpy for generating pages to transfer
> - vmsplice with SPLICE_F_GIFT sending over named pipe (in a loop)
> Receiving process:
> - Create mmaped file to receive the pages
> - splice with SPLICE_F_MOVE receiving from named pipe (in a loop)

As the splice(2) man page says SPLICE_F_MOVE is currently a no-op.

> I have seen the SPLICE_F_MOVE being used on steal ops from the
> 'pipebuffer', but I couldn't find a way to call it from splice.
>
> Questions:
> It does what I think it does? (reassign memory pages from a process to
> another)

> If so, does page gifting still works?
> If so, is there a basic recipe to test it's workings?

What is the end goal?

It is easy to transfer pages using shared memory (see shm_open(3) and
related API), so why mess with splice?

Thanks,
Miklos