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: <20250331203014.5108200c@gandalf.local.home>
Date: Mon, 31 Mar 2025 20:30:14 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: 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

On Mon, 31 Mar 2025 17:11:46 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> I thought you did that already for the user mappings - don't use you
> remap_pfn_range()?

No, that's not what is done. The normal buffer is split among several
sub-buffers (usually one page in size each, but can also be a power of two
pages), and those pages are allocated via alloc_page() and are not
contiguous. Which is why the mapping to user space creates an array of
struct page pointers and then calls vm_insert_pages().

For the contigous physical memory, then yeah, we can simply use
vm_iomap_memory().

> So if you don't treat this as some kind of 'page' or 'folio' thing,
> then the proper function is actually flush_cache_range().
> 
> I actually suspect that if you treat things just as an arbitrary range
> of memory, it might simplify things in general.

Ah, yeah. That's the function I was looking for.

> Of course, I would expect the same to be true of the page/folio cases,
> so I don't think using flush_cache_range() should be any worse, but I
> *could* imagine that it's bad in a different way ;)

At least we can say we covered those other archs, and if a bug is reported,
then all that would need to be fixed is the flush_cache_range()
implementation ;-)

-- Steve