linux-kernel - Re: [PATCH v2 1/2] tracing: ring-buffer: Have the ring buffer code do the vmap of physical memory

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Message-ID: <Z-u3jW1zdqic9IKy@gmail.com>
Date: Tue, 1 Apr 2025 11:53:17 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Steven Rostedt <rostedt@...dmis.org>, Jann Horn <jannh@...gle.com>,
	linux-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>,
	Vincent Donnefort <vdonnefort@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
	Kees Cook <kees@...nel.org>, Tony Luck <tony.luck@...el.com>,
	"Guilherme G. Piccoli" <gpiccoli@...lia.com>,
	linux-hardening@...r.kernel.org,
	Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2 1/2] tracing: ring-buffer: Have the ring buffer code
 do the vmap of physical memory


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Mon, 31 Mar 2025 at 18:40, Steven Rostedt <rostedt@...dmis.org> wrote:
> > >
> > > I think 'pfn' was introduced as a name ong long ago because it was
> > > what the alpha architecture used in the VM documentation. It probably
> > > predates that too, but it's where I got it from, iirc.
> > >
> >
> > It is old, as I remember using it when I first started Linux kernel
> > development back in 1998.
> 
> So I did the alpha port in '95, but I meant that the 'page frame
> number' as a name may well have roots that go much further back. I
> only know it from the alpha architecture docs.
> 
> Google only seems to find the modern Linux use, but I wouldn't be
> surprised if the alpha architects got it from some much older use (I
> would suspect VMS).

*Technically*, for those of us with a weakness for Git archeology, the 
'PFN' term was first introduced in Linux with the MIPS port, which went 
upstream via Linux 1.1.45, released on 1994/04/06, and had this in 
include/asm-mips/mipsregs.h:

  +/*
  + * Compute a vpn/pfn entry for EntryHi register
  + */
  +#define VPN(addr,pagesizeshift) ((addr) & ~((1 << (pagesizeshift))-1))
  +#define PFN(addr,pagesizeshift) (((addr) & ((1 << (pagesizeshift))-1)) << 6)

... while your Alpha port went upstream via 1.1.57, released on 
1994/10/24, and the first mention of 'PFN' was in arch/alpha/mm/init.c, 
introduced as part of 1.1.82, released 1995/01/16, almost a year later:

  +unsigned long paging_init(unsigned long start_mem, unsigned long end_mem)
  ..
  +               unsigned long pfn, nr;
  +               if (cluster->usage & 1)
  +                       continue;
  +               pfn = cluster->start_pfn;
  +               nr = cluster->numpages;

So you don't appear to have dibs on the PFN name within Linux, although 
I suspect MIPS and Alpha had it from a similar background. :-)

By the way, some quick review feedback on that patch: there should be 
extra newlines after variable declarations - clearly the author is not 
a good kernel programm... oh, never mind.

Thanks,

	Ingo