linux-kernel - Re: [PATCH v5 12/14] mm: add config option for clearing page-extents

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Message-ID: <2c6db237-3d64-43f5-8a20-168be5b248c8@redhat.com>
Date: Fri, 11 Jul 2025 21:14:50 +0200
From: David Hildenbrand <david@...hat.com>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
 akpm@...ux-foundation.org, bp@...en8.de, dave.hansen@...ux.intel.com,
 hpa@...or.com, mingo@...hat.com, mjguzik@...il.com, luto@...nel.org,
 peterz@...radead.org, acme@...nel.org, namhyung@...nel.org,
 tglx@...utronix.de, willy@...radead.org, raghavendra.kt@....com,
 boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v5 12/14] mm: add config option for clearing page-extents

On 11.07.25 19:25, Ankur Arora wrote:
> 
> David Hildenbrand <david@...hat.com> writes:
> 
>> On 10.07.25 02:59, Ankur Arora wrote:
>>> Add CONFIG_CLEAR_PAGE_EXTENT to allow clearing of page-extents
>>> where architecturally supported.
>>> This is only available with !CONFIG_HIGHMEM because the intent is to
>>> use architecture support to clear contiguous extents in a single
>>> operation (ex. via FEAT_MOPS on arm64, string instructions on x86)
>>> which excludes any possibility of interspersing kmap()/kunmap().
>>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>>> ---
>>>    mm/Kconfig | 9 +++++++++
>>>    1 file changed, 9 insertions(+)
>>> diff --git a/mm/Kconfig b/mm/Kconfig
>>> index 781be3240e21..a74a5e02de28 100644
>>> --- a/mm/Kconfig
>>> +++ b/mm/Kconfig
>>> @@ -910,6 +910,15 @@ config NO_PAGE_MAPCOUNT
>>>      endif # TRANSPARENT_HUGEPAGE
>>>    +config CLEAR_PAGE_EXTENT
>>> +	def_bool y
>>> +	depends on !HIGHMEM && ARCH_HAS_CLEAR_PAGES
>>> +	depends on TRANSPARENT_HUGEPAGE || HUGETLBFS
>>> +
>>> +	help
>>> +	  Use architectural support for clear_pages() to zero page-extents.
>>> +	  This is likely to be faster than zeroing page-at-a-time.
>>> +
>>
>> IIRC, adding a help text will make this option be configurable by the user, no?
> 
> I tried changing the value in the generated .config by hand and that
> reverted back to the computed value. So, I think this isn't configurable
> by the user.

Yes, I misremembered and the help text confused me.

So yes, the help text in that case (internal entries) is uncommon (no 
need to guide the user when there are no options :) ), but feel free to 
keep it for documentation purposes.

(e.g., HAVE_64BIT_ALIGNED_ACCESS does that)

Maybe we should call it similarly HAVE_... to make it clearer that there 
are really no options.

But I'm bad at #kconfig, so whatever you prefer.

-- 
Cheers,

David / dhildenb