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

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Message-ID: <87ecumms40.fsf@oracle.com>
Date: Fri, 11 Jul 2025 12:42:39 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, 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


David Hildenbrand <david@...hat.com> writes:

> On 11.07.25 19:32, 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>
>>>> ---
>>>
>>> Staring at the next patch, I think this can easily be squashed into the next
>>> patch where you add actual MM core support.
>> I wanted to do this in a separate patch to explicitly document what the
>> responsibility of the interface provided by the architecture is.
>> That said, the commit message didn't actually do a good job of doing
>> that :D.
>> Copying the more detailed commit message from my reply to Andrew,
>> one important part of the clear_pages() is that it be interruptible
>> because clear_pages_resched() implicitly depends on it.
>>
>>> This is only enabled with !CONFIG_HIGHMEM because the intent is
>>> to use architecture support to clear contiguous extents in a
>>> single interruptible operation (ex. via FEAT_MOPS on arm64,
>>> string instructions on x86).
>>
>>> Given that we might be zeroing the whole extent with a single
>>> instruction, this excludes any possibility of constructing
>>> intermediate HIGHMEM maps.
>> Do you think it is best documented in the next patch in a comment
>> instead?
>
> I would just add + document it as part of the next patch.
>
> Looking at the bigger picture now, you introduce
>
> 	ARCH_HAS_CLEAR_PAGES
>
> To say whether an architecture provides clear_pages().
>
> Now we want to conditionally use that to optimize folio_zero_user().
>
> Remind me, why do we want to glue this to THP / HUGETLBFS only? I would assume
> that the code footprint is rather small, and the systems out there that are
> compiled with ARCH_HAS_CLEAR_PAGES but without THP / HUGETLBFS are rather ...
> rare (mostly 32BIT x86 only).

A failure of imagination, I guess. I couldn't think of a way anyone
would have to operate on contiguous pages without also having THP or
HUGETLBFS.

And, for the cases where we want to operate on kernel ranges, we could
directly use clear_pages().
(Though that needs some work to add a generic wrapper.)

--
ankur