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

From: Will Deacon <will@kernel.org>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: 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>,
	ardb@kernel.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] Revert "mm/pgtable: add stubs for {pmd/pub}_{set/clear}_huge"
Date: Mon, 19 Jul 2021 11:49:19 +0100	[thread overview]
Message-ID: <20210719104918.GA6440@willie-the-truck> (raw)
In-Reply-To: <20210717183108.Horde.NxzyIKsItUWF4WlYgwgptg1@messagerie.c-s.fr>

On Sat, Jul 17, 2021 at 06:31:08PM +0200, Christophe Leroy wrote:
> Jonathan Marek <jonathan@marek.ca> a écrit :
> 
> > c742199a breaks arm64 for some configs because it stubs out functions which
> > should not have been stubbed out.
> > 
> > With 4K pages and ARM64_VA_BITS_39=y, the kernel crashes early on unmapped
> > 1G pages in the linear map caused by pud_set_huge() in alloc_init_pud()
> > being stubbed out. Reverting c742199a fixes the crash.
> 
> This patch is there for a reason. Reverting it will break some other config.

Which config? Not reverting it breaks arm64.

> The fix needs to allow it work with all platforms.

Hmm, there was already a report that selftests were failing with this
change:

https://lore.kernel.org/linux-arm-kernel/CAMuHMdXShORDox-xxaeUfDW3wx2PeggFSqhVSHVZNKCGK-y_vQ@mail.gmail.com/

That was a week ago and doesn't seem to have progressed, so I'm inclined to
revert this if it's not resolved this week as we usually use -rc3 as a base
for queuing patches for the next release.

> I don't understand, why does arm64 needs these PUD helpers when it defines
> __PAGETABLE_PUD_FOLDED ?

Probably for the huge vmap code; see arch_vmap_pXd_supported(). That also
lines up with the failing selftests afaict.

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).

Will

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