Re: [PATCH] mm/thp: Make ARCH_ENABLE_SPLIT_PMD_PTLOCK dependent on PGTABLE_LEVELS > 2

From: Anshuman Khandual <anshuman.khandual@arm.com>
To: Oscar Salvador <osalvador@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
	linux-mm@kvack.org, akpm@linux-foundation.org,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	x86@kernel.org, linux-s390@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/thp: Make ARCH_ENABLE_SPLIT_PMD_PTLOCK dependent on PGTABLE_LEVELS > 2
Date: Mon, 10 May 2021 15:46:41 +0530	[thread overview]
Message-ID: <a6ebcb5b-1fd6-d03f-cfb5-52e32b60899a@arm.com> (raw)
In-Reply-To: <20210510101006.GB22664@linux>

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.