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

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Message-ID: <20250215114540.7203feaf@batman.local.home>
Date: Sat, 15 Feb 2025 11:45:40 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Vincent Donnefort <vdonnefort@...gle.com>
Subject: Re: [PATCH] tracing: Do not allow mmap() of persistent ring buffer

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

> On Sun, 16 Feb 2025 00:37:02 +0900
> Masami Hiramatsu (Google) <mhiramat@...nel.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