linux-kernel - Re: [PATCH v1 03/11] x86/mm/pat: introduce pfnmap_track() and pfnmap

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Message-ID: <632e9033-047d-4b40-8677-cd2c5da286dc@lucifer.local>
Date: Mon, 28 Apr 2025 19:58:14 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-trace-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tursulin@...ulin.net>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v1 03/11] x86/mm/pat: introduce pfnmap_track() and
 pfnmap_untrack()

On Mon, Apr 28, 2025 at 07:12:11PM +0200, David Hildenbrand wrote:
>
> > >
> > > +int pfnmap_track(unsigned long pfn, unsigned long size, pgprot_t *prot)
> > > +{
> > > +	const resource_size_t paddr = (resource_size_t)pfn << PAGE_SHIFT;
> > > +
> > > +	return reserve_pfn_range(paddr, size, prot, 0);
> >
> > Nitty, but a pattern established by Liam which we've followed consistently
> > in VMA code is to prefix parameters that might be less than obvious,
> > especially boolean parameters, with a comment naming the parameter, e.g.:
> > > 	return reserve_pfn_range(paddr, size, prot, /*strict_prot=*/0);
>
> Not sure I like that. But as this parameter goes away patch #8, I'll leave
> it as is in this patch and not start a bigger discussion on better
> alternatives (don't use these stupid boolean variables ...) ;)
>
> [...]
>
> > > +
> > > +/**
> > > + * pfnmap_track - track a pfn range
> >
> > To risk sounding annoyingly pedantic and giving the kind of review that is
> > annoying, this really needs to be expanded, I think perhaps this
> > description is stating the obvious :)
> >
> > To me the confusing thing is that the 'generic' sounding pfnmap_track() is
> > actually PAT-specific, so surely the description should give a brief
> > overview of PAT here, saying it's applicable on x86-64 etc. etc.
> >
> > I'm not sure there's much use in keeping this generic when it clearly is
> > not at this point?
>
> Sorry, is your suggestion to document more PAT stuff or what exactly?
>
> As you know, I'm a busy man ... so instructions/suggestions please :)

Haha sure, I _think_ the model here is to have a brief summary then underneath a
more detailed explanation, so that could be:

	This address range is requested to be 'tracked' by a hardware
	implementation allowing fine-grained control of memory attributes at
	page level granularity.

	This allows for fine-grained control of memory cache behaviour. Tracking
	memory this way is persisted across VMA split and merge.

	Currently there is only one implementation for this - x86 Page Attribute
	Table (PAT). See Documentation/arch/x86/pat.rst for more details.

>
> >
> > > + * @pfn: the start of the pfn range
> > > + * @size: the size of the pfn range
> >
> > In what units? Given it's a pfn range it's a bit ambiguous as to whether it
> > should be expressed in pages/bytes.
>
> Agreed. It's bytes. (not my favorite here, but good enough)

Ack, definitely need to spell it out here! Cheers :)

>
>
> --
> Cheers,
>
> David / dhildenb
>