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

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Date:   Fri, 20 Aug 2021 14:25:12 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Daniel Axtens <dja@...ens.net>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        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

Daniel Axtens <dja@...ens.net> writes:
> 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.)

Yeah. I rewrote the change log a bit to make it clear this is a bug fix,
not a harmless cleanup.

cheers


  powerpc: Re-enable ARCH_ENABLE_SPLIT_PMD_PTLOCK
  
  Commit 66f24fa766e3 ("mm: drop redundant ARCH_ENABLE_SPLIT_PMD_PTLOCK")
  broke PMD split page table lock for powerpc.
  
  It selects the non-existent config ARCH_ENABLE_PMD_SPLIT_PTLOCK in
  arch/powerpc/platforms/Kconfig.cputype, but clearly intended to
  select ARCH_ENABLE_SPLIT_PMD_PTLOCK (notice the word swapping!), as
  that commit did for all other architectures.
  
  Fix it by selecting the correct symbol ARCH_ENABLE_SPLIT_PMD_PTLOCK.
  
  Fixes: 66f24fa766e3 ("mm: drop redundant ARCH_ENABLE_SPLIT_PMD_PTLOCK")
  Cc: stable@...r.kernel.org # v5.13+
  Signed-off-by: Lukas Bulwahn <lukas.bulwahn@...il.com>
  Reviewed-by: Daniel Axtens <dja@...ens.net>
  [mpe: Reword change log to make it clear this is a bug fix]
  Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
  Link: https://lore.kernel.org/r/20210819113954.17515-3-lukas.bulwahn@gmail.com