Re: [PATCH] Revert "mm/pgtable: add stubs for {pmd/pub}_{set/clear}_huge"

From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Will Deacon <will@kernel.org>
Cc: Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Jonathan Marek <jonathan@marek.ca>,
	Mike Rapoport <rppt@kernel.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Ard Biesheuvel <ardb@kernel.org>
Subject: Re: [PATCH] Revert "mm/pgtable: add stubs for {pmd/pub}_{set/clear}_huge"
Date: Tue, 20 Jul 2021 17:14:33 +0200	[thread overview]
Message-ID: <20210720171433.Horde.K_Ueg9cpbL1nEKt7zO9dTg6@messagerie.c-s.fr> (raw)
In-Reply-To: <20210720112704.GA8419@willie-the-truck>

Will Deacon <will@kernel.org> a écrit :

> On Mon, Jul 19, 2021 at 06:58:43PM +0200, Christophe Leroy wrote:
>> Ard Biesheuvel <ardb@kernel.org> a écrit :
>> > On Mon, 19 Jul 2021 at 17:45, Christophe Leroy
>> > <christophe.leroy@csgroup.eu> wrote:
>> > > Ard Biesheuvel <ardb@kernel.org> a écrit :
>> > > > On Mon, 19 Jul 2021 at 15:58, Ard Biesheuvel <ardb@kernel.org> wrote:
>> > > >> On Mon, 19 Jul 2021 at 12:49, Will Deacon <will@kernel.org> wrote:
>> > > >> > For 4k pages with 3 levels of walk, we want to be able to use
>> > > 1GB mappings
>> > > >> > at level 1 (pgd), 2MB mappings at level 2 (pmd) as well as  
>> the usual 4k
>> > > >> > page mappings at level 3 (pte).
>> > > >> >
>> > > >>
>> > > >> Yes, we appear to use PUDs for 4k pages kernel regardless of whether
>> > > >> they are folded into PGDs to refer to level 1/1GB block mappings.
>> > >
>> > > And that's probably the source of the misunderstanding, because a 3
>> > > level architecture is supposed to only have pgd, pmd and pud.
>> > >
>> >
>> > Is this requirement documented somewhere? Having folded PUDs does not
>> > necessarily mean that PUDs don't exist, but simply that PUDs and PGDs
>> > are the same.
>>
>> If you look at linux/pgtable.h you can see that p4d_set_huge and
>> p4d_clear_huge are only for 5 level MMUs. Why shouldn't the same principle
>> apply to PUD and PMD ?
>
> Prior to your patch, you could make the opposite argument. Why are
> p4x_{set_clear}_huge() guarded? Those were introduced after the others
> as well.
>
> Reverting looks easiest for now. Let's do that so we can figure this ou
> properly without breaking mainline in the meantime. That also means you
> can enjoy your holiday without looking at email on your phone!

No need to be that extreme. See my comment to the revert patch, all  
you need to do is to add the two missing functions in 8xx.c as part of  
the patch reverting the add of the stubs in linux/pgtable.h

Christophe

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