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

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Message-ID: <fc8117d9-57f8-4c28-9c46-328e4a3c4613@redhat.com>
Date: Mon, 28 Apr 2025 19:12:11 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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()

>>
>> +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 :)

> 
>> + * @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)

-- 
Cheers,

David / dhildenb