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

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Message-ID: <87ldowovm1.fsf@oracle.com>
Date: Thu, 10 Jul 2025 09:31:50 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, x86@...nel.org, david@...hat.com, 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


Andrew Morton <akpm@...ux-foundation.org> writes:

> On Wed,  9 Jul 2025 17:59:24 -0700 Ankur Arora <ankur.a.arora@...cle.com> wrote:
>
>> 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().
>
> I'm feeling dumb.  What does the use of a string instruction have to do
> with kmap/kunmap?

Sorry, that might have been unnecessarily inscrutable. How about
something like this:

 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.

--
ankur