linux-kernel - Re: [PATCH v2 2/2] powerpc: rectify selection to ARCH_ENABLE_SPLIT_PMD

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Date:   Fri, 20 Aug 2021 10:03:08 +1000
From:   Daniel Axtens <dja@...ens.net>
To:     Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Neuling <mikey@...ling.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        kvm-ppc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Cc:     Lukas Bulwahn <lukas.bulwahn@...il.com>,
        kernel-janitors@...r.kernel.org, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] powerpc: rectify selection to
 ARCH_ENABLE_SPLIT_PMD_PTLOCK

Lukas Bulwahn <lukas.bulwahn@...il.com> writes:

> Commit 66f24fa766e3 ("mm: drop redundant ARCH_ENABLE_SPLIT_PMD_PTLOCK")
> selects the non-existing config ARCH_ENABLE_PMD_SPLIT_PTLOCK in
> ./arch/powerpc/platforms/Kconfig.cputype, but clearly it intends to select
> ARCH_ENABLE_SPLIT_PMD_PTLOCK here (notice the word swapping!), as this
> commit does select that for all other architectures.
>
> Rectify selection to ARCH_ENABLE_SPLIT_PMD_PTLOCK instead.
>

Yikes, yes, 66f24fa766e3 does seem to have got that wrong. It looks like
that went into 5.13.

I think we want to specifically target this for stable so that we don't
lose the perfomance and scalability benefits of split pmd ptlocks:

Cc: stable@...r.kernel.org # v5.13+

(I don't think you need to do another revision for this, I think mpe
could add it when merging.)

I tried to check whether we accidentally broke SPLIT_PMD_PTLOCKs while
they were disabled:

 - There hasn't been any change to the pgtable_pmd_page_ctor or _dtor
   prototypes, and we haven't made any relevant changes to any of the
   files in arch/powerpc that called it.

 - I checked out v5.13 and powerpc/merge, applied this patch, built a
   pseries_le_defconfig and boot tested it in qemu. It didn't crash on
   boot or with /bin/sh and some shell commands, but I didn't exactly
   stress test the VM subsystem either.

This gives me some confidence it's both good for powerpc and stable-worthy.

Overall:
Reviewed-by: Daniel Axtens <dja@...ens.net>

Kind regards,
Daniel

> Fixes: 66f24fa766e3 ("mm: drop redundant ARCH_ENABLE_SPLIT_PMD_PTLOCK")
> Signed-off-by: Lukas Bulwahn <lukas.bulwahn@...il.com>
> ---
>  arch/powerpc/platforms/Kconfig.cputype | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
> index 6794145603de..a208997ade88 100644
> --- a/arch/powerpc/platforms/Kconfig.cputype
> +++ b/arch/powerpc/platforms/Kconfig.cputype
> @@ -98,7 +98,7 @@ config PPC_BOOK3S_64
>  	select PPC_HAVE_PMU_SUPPORT
>  	select HAVE_ARCH_TRANSPARENT_HUGEPAGE
>  	select ARCH_ENABLE_HUGEPAGE_MIGRATION if HUGETLB_PAGE && MIGRATION
> -	select ARCH_ENABLE_PMD_SPLIT_PTLOCK
> +	select ARCH_ENABLE_SPLIT_PMD_PTLOCK
>  	select ARCH_ENABLE_THP_MIGRATION if TRANSPARENT_HUGEPAGE
>  	select ARCH_SUPPORTS_HUGETLBFS
>  	select ARCH_SUPPORTS_NUMA_BALANCING
> -- 
> 2.26.2