linux-kernel - Re: [PATCH] mm/thp: Make ARCH_ENABLE_SPLIT_PMD_PTLOCK dependent on PGTABLE

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Date:   Mon, 10 May 2021 15:46:41 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Peter Zijlstra <peterz@...radead.org>, linux-mm@...ck.org,
        akpm@...ux-foundation.org, Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/thp: Make ARCH_ENABLE_SPLIT_PMD_PTLOCK dependent on
 PGTABLE_LEVELS > 2



On 5/10/21 3:40 PM, Oscar Salvador wrote:
> On Mon, May 10, 2021 at 03:36:29PM +0530, Anshuman Khandual wrote:
>>
>>
>> On 5/10/21 2:23 PM, Peter Zijlstra wrote:
>>> On Mon, May 10, 2021 at 10:05:45AM +0530, Anshuman Khandual wrote:
>>>> -	select ARCH_ENABLE_SPLIT_PMD_PTLOCK if X86_64 || X86_PAE
>>>> +	select ARCH_ENABLE_SPLIT_PMD_PTLOCK if (PGTABLE_LEVELS > 2) && (X86_64 || X86_PAE)
>>>
>>> It's still very early on a Monday, but IIRC this new condition is
>>> identical to the pre-existing one.
>>
>> Did not get it, could you please elaborate ?
> 
> When using x86_PAE, you must have more than two pgtable levels, right?
> And not speaking of x86_64.

arch/x86/Kconfig..

config PGTABLE_LEVELS
        int
        default 5 if X86_5LEVEL
        default 4 if X86_64
        default 3 if X86_PAE
        default 2

Both X86_PAE and X86_64 will always have page table levels > 2 ? But
regardless, it might be still useful to assert (PGTABLE_LEVELS > 2)
before selecting ARCH_ENABLE_SPLIT_PMD_PTLOCK.