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

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Message-ID: <87qzymocok.fsf@oracle.com>
Date: Fri, 11 Jul 2025 10:32:59 -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 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?

--
ankur