Re: [PATCH v11 2/5] ring-buffer: Introducing ring-buffer mapping functions

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2024-01-11 11:17, Vincent Donnefort wrote:
> In preparation for allowing the user-space to map a ring-buffer, add
> a set of mapping functions:
> 
>    ring_buffer_{map,unmap}()
>    ring_buffer_map_fault()
> 
> And controls on the ring-buffer:
> 
>    ring_buffer_map_get_reader()  /* swap reader and head */
> 
> Mapping the ring-buffer also involves:
> 
>    A unique ID for each subbuf of the ring-buffer, currently they are
>    only identified through their in-kernel VA.
> 
>    A meta-page, where are stored ring-buffer statistics and a
>    description for the current reader
> 

Hi Vincent,

The LTTng kernel tracer has supported mmap'd buffers for nearly 15 years [1],
and has a lot of similarities with this patch series.

LTTng has the notion of "subbuffer id" to allow atomically exchanging a
"reader" extra subbuffer with the subbuffer to be read. It implements
"get subbuffer" / "put subbuffer" ioctls to allow the consumer (reader)
to move the currently read subbuffer position. [2]

It would not hurt to compare your approach to LTTng and highlight
similarities/differences, and the rationale for the differences.

Especially when it comes to designing kernel ABIs, it's good to make sure
that all bases are covered, because those choices will have lasting impacts.

Thanks,

Mathieu

[1] https://git.lttng.org/?p=lttng-modules.git;a=blob;f=src/lib/ringbuffer/ring_buffer_mmap.c
[2] https://git.lttng.org/?p=lttng-modules.git;a=blob;f=src/lib/ringbuffer/ring_buffer_vfs.c

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com