Re: [PATCH] tracing: Do not allow mmap() of persistent ring buffer

Steven Rostedt
2025-02-15T16:45:42Z

On Sat, 15 Feb 2025 11:21:53 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Sun, 16 Feb 2025 00:37:02 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
>
> > My concern is related to the fixes policy. If this is a "fix", we will
> > backport the new "disables mmap on persistent ring buffer" limitation
> > to the stable kernel (that was not documented previously.)
> >
> > However, from the user point of view, "mmap() ring buffers" is already
> > supported (although it did not work on stable kernel for now). Thus I think
> > the "Fix" is expected as "fixing mmap() persistent ring buffer".
>
> This only disables mmapping of the persistent ring buffer. Other ring
> buffers can be mapped. We never supported mmapping the persistent ring
> buffer. Even in stable kernels, if you mmap it, it will crash just like
> it does now. Thus, this doesn't cause any regressions. It's a fix even
> for stable kernels.
>
> Or did the virt_to_page() change recently where that wasn't the case?
>

Although the fixes tag is wrong. As the persistent ring buffer didn't
even exist then. It should be:

Fixes: 9b7bdf6f6ece6 ("tracing: Have trace_printk not use binary prints if boot buffer")

As that's what added the BOOT flag and is in the same kernel version
that added the persistent ring buffer.

-- Steve