linux-kernel - Re: [for-linus][PATCH 0/3] tracing: Fixes for v6.8

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Date: Sun, 10 Mar 2024 14:36:40 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: David Laight <David.Laight@...LAB.COM>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>
Subject: Re: [for-linus][PATCH 0/3] tracing: Fixes for v6.8

On Sun, 10 Mar 2024 18:16:06 +0000
David Laight <David.Laight@...LAB.COM> wrote:

> ...
> >   Another issue that was brought up is that the trace_seq buffer is
> >   also based on PAGE_SIZE even though it is not tied to the architecture
> >   limit like the ring buffer sub-buffer is. Having it be 64K * 2 is
> >   simply just too big and wasting memory on systems with 64K page sizes.
> >   It is now hardcoded to 8K which is what all other architectures with
> >   4K PAGE_SIZE has.  
> 
> Does Linux use a 2k PAGE_SIZE on any architectures?
> IIRC m68k hardware has a 2k page, but Linux might always pair them.
> A 2k page might (or might not) cause grief.
> 

The trace_seq is just a buffer to build up the event output string. The
ring buffer sub-buffer is set to page size. For trace_marker, it is
still limited to the size of the ring buffer sub-buffer. If the
sub-buffer is only 2K, the trace_marker write will be broken up by less
than 2K.

The problem that is being fixed here had nothing to do with the limited
size of the resources. The issue was actually the opposite. On PowerPC,
the PAGE_SIZE being 64K allowed the strings to be that big too. And
what broke was that it was passed to a vsprintf(s, "%.*s", len, str);
where the len was greater than 32K and that caused a warning as the
precision of "%.*s" has a max of signed short.

2K PAGE_SIZE will still just "work".

-- Steve