tracing: Have memmapped ring buffer use ioctl of "R" range 0x20-2F

To prevent conflicts with other ioctl numbers to allow strace to have an
idea of what is happening, add the range of ioctls for the

tracing: Have memmapped ring buffer use ioctl of "R" range 0x20-2F

To prevent conflicts with other ioctl numbers to allow strace to have an
idea of what is happening, add the range of ioctls for the trace buffer
mapping from _IO("T", 0x1) to the range of "R" 0x20 - 0x2F.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]/
Link: https://lore.kernel.org/linux-trace-kernel/[email protected]/

Cc: Jonathan Corbet <[email protected]>
Fixes: cf9f0f7c4c5bb ("tracing: Allow user-space mapping of the ring-buffer")
Link: https://lore.kernel.org/[email protected]
Reported-by: "Dmitry V. Levin" <[email protected]>
Reviewed-by: Mathieu Desnoyers <[email protected]>
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

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tracing: Allow user-space mapping of the ring-buffer

Currently, user-space extracts data from the ring-buffer via splice,
which is handy for storage or network sharing. However, due to splice
limita

tracing: Allow user-space mapping of the ring-buffer

Currently, user-space extracts data from the ring-buffer via splice,
which is handy for storage or network sharing. However, due to splice
limitations, it is imposible to do real-time analysis without a copy.

A solution for that problem is to let the user-space map the ring-buffer
directly.

The mapping is exposed via the per-CPU file trace_pipe_raw. The first
element of the mapping is the meta-page. It is followed by each
subbuffer constituting the ring-buffer, ordered by their unique page ID:

* Meta-page -- include/uapi/linux/trace_mmap.h for a description
* Subbuf ID 0
* Subbuf ID 1
...

It is therefore easy to translate a subbuf ID into an offset in the
mapping:

reader_id = meta->reader->id;
reader_offset = meta->meta_page_size + reader_id * meta->subbuf_size;

When new data is available, the mapper must call a newly introduced ioctl:
TRACE_MMAP_IOCTL_GET_READER. This will update the Meta-page reader ID to
point to the next reader containing unread data.

Mapping will prevent snapshot and buffer size modifications.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

CC: <[email protected]>
Signed-off-by: Vincent Donnefort <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

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ring-buffer: Introducing ring-buffer mapping functions

In preparation for allowing the user-space to map a ring-buffer, add
a set of mapping functions:

ring_buffer_{map,unmap}()

And controls on

ring-buffer: Introducing ring-buffer mapping functions

In preparation for allowing the user-space to map a ring-buffer, add
a set of mapping functions:

ring_buffer_{map,unmap}()

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

The linear mapping exposes the meta-page, and each subbuf of the
ring-buffer, ordered following their unique ID, assigned during the
first mapping.

Once mapped, no subbuf can get in or out of the ring-buffer: the buffer
size will remain unmodified and the splice enabling functions will in
reality simply memcpy the data instead of swapping subbufs.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

CC: <[email protected]>
Signed-off-by: Vincent Donnefort <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

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