linux-kernel - Re: [PATCH v5 02/15] ring-buffer: Page size per ring buffer

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Message-ID: <20231221091947.3b35d5d8@gandalf.local.home>
Date: Thu, 21 Dec 2023 09:19:47 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: David Laight <David.Laight@...LAB.COM>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-trace-kernel@...r.kernel.org" <linux-trace-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>, Tzvetomir Stoyanov
 <tz.stoyanov@...il.com>, Vincent Donnefort <vdonnefort@...gle.com>, Kent
 Overstreet <kent.overstreet@...il.com>
Subject: Re: [PATCH v5 02/15] ring-buffer: Page size per ring buffer

On Thu, 21 Dec 2023 09:17:55 +0000
David Laight <David.Laight@...LAB.COM> wrote:

> > Unfortunately, it has to be PAGE_SIZE (and for now it's a power of 2 to
> > make masking easy). It's used for splice and will also be used for memory
> > mapping with user space.  
> 
> Perhaps then the sysctl to set the size should be powers of 4k

It's not a sysctl but a file in tracefs

> with a minimum size of PAGE_SIZE.
> Then you don't have to know the page size when setting things up.

The user shouldn't need to know either. But the size of the sub-buffer
limits the biggest size of an event, so the user only needs to make sure
the sub-buffer is bigger than their biggest event.

> 
> I'm also guessing that no Linux kernels have a PAGE_SIZE of 2k?
> IIRC some old mmu (maybe 68020 era) used 2k pages.

I think 1kb units is perfectly fine (patch 15 changes to kb units). The
interface says its to define the minimal size of the sub-buffer, not the
actual size.

-- Steve