tracing: Add boot-time backup of persistent ring buffer

tracing: Add boot-time backup of persistent ring buffer

Currently, the persistent ring buffer instance needs to be read before
using it. This means we have to wait for boot up user space and dump
the persistent ring buffer. However, in that case we can not start
tracing on it from the kernel cmdline.

To solve this limitation, this adds an option which allows to create
a trace instance as a backup of the persistent ring buffer at boot.
If user specifies trace_instance=<BACKUP>=<PERSIST_RB> then the
<BACKUP> instance is made as a copy of the <PERSIST_RB> instance.

For example, the below kernel cmdline records all syscalls, scheduler
and interrupt events on the persistent ring buffer `boot_map` but
before starting the tracing, it makes a `backup` instance from the
`boot_map`. Thus, the `backup` instance has the previous boot events.

'reserve_mem=12M:4M:trace trace_instance=boot_map@trace,syscalls:*,sched:*,irq:* trace_instance=backup=boot_map'

As you can see, this just make a copy of entire reserved area and
make a backup instance on it. So you can release (or shrink) the
backup instance after use it to save the memory usage.

  /sys/kernel/tracing/instances # free
                total        used        free      shared  buff/cache   available
  Mem:        1999284       55704     1930520       10132       13060     1914628
  Swap:             0           0           0
  /sys/kernel/tracing/instances # rmdir backup/
  /sys/kernel/tracing/instances # free
                total        used        free      shared  buff/cache   available
  Mem:        1999284       40640     1945584       10132       13060     1929692
  Swap:             0           0           0

Note: since there is no reason to make a copy of empty buffer, this
backup only accepts a persistent ring buffer as the original instance.
Also, since this backup is based on vmalloc(), it does not support
user-space mmap().

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/176377150002.219692.9425536150438129267.stgit@devnote2
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>