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

From: LEROY Christophe <christophe.leroy@csgroup.eu>
To: Will Deacon <will@kernel.org>
Cc: Ard Biesheuvel <ardb@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Mike Rapoport <rppt@kernel.org>,
	Jonathan Marek <jonathan@marek.ca>,
	Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] Revert "mm/pgtable: add stubs for {pmd/pub}_{set/clear}_huge"
Date: Mon, 2 Aug 2021 13:39:00 +0200	[thread overview]
Message-ID: <cfd1e226-61d5-cbf2-1137-40c30a59d5d7@csgroup.eu> (raw)
In-Reply-To: <20210720112704.GA8419@willie-the-truck>

Le 20/07/2021 à 13:27, Will Deacon 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.
> 

Why are they guarded ? Probably because when they aren't, with ARM64 
defconfig we get:

   LD      vmlinux.o
   MODPOST vmlinux.symvers
   MODINFO modules.builtin.modinfo
   GEN     modules.builtin
   LD      .tmp_vmlinux.kallsyms1
aarch64-linux-gnu-ld: Unexpected GOT/PLT entries detected!
aarch64-linux-gnu-ld: Unexpected run-time procedure linkages detected!
aarch64-linux-gnu-ld: mm/vmalloc.o: in function `vunmap_p4d_range':
/home/chleroy/linux-powerpc/mm/vmalloc.c:385: undefined reference to 
`p4d_clear_huge'
aarch64-linux-gnu-ld: /home/chleroy/linux-powerpc/mm/vmalloc.c:385: 
undefined reference to `p4d_clear_huge'
make: *** [Makefile:1177: vmlinux] Error 1

Christophe

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