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

[Ankur Arora <ankur.a.arora@oracle.com>]

David Hildenbrand <david@redhat.com> writes:

> On 11.07.25 19:25, Ankur Arora wrote:
>> David Hildenbrand <david@redhat.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@oracle.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.

Actually a prefix is a good idea.

Btw, the natural follow-up (to me anyway) to this series would be to
also do this for the copy path.

From some quick tests on x86, the performance improvement was similar.

So, maybe it could just be something like: USE_CONTIG_PAGES.

But that needs a verb somewhere.

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

Not much better, as you can see.

--
ankur