linux-kernel - Re: [PATCH v4 13/13] x86/folio_zero

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Message-ID: <1c0d3994-1397-4bc6-bb55-9c26acdcb477@intel.com>
Date: Mon, 16 Jun 2025 07:44:13 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, x86@...nel.org
Cc: 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, jon.grimm@....com, bharata@....com,
 raghavendra.kt@....com, boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v4 13/13] x86/folio_zero_user: Add multi-page clearing

On 6/15/25 22:22, Ankur Arora wrote:
> Override the common code version of folio_zero_user() so we can use
> clear_pages() to do multi-page clearing instead of the standard
> page-at-a-time clearing.

I'm not a big fan of the naming in this series.

To me multi-page means "more than one 'struct page'". But this series is
clearly using multi-page clearing to mean clearing >PAGE_SIZE in one
clear. But oh well.

The second problem with where this ends up is that none of the code is
*actually* x86-specific. The only thing that x86 provides that's
interesting is a clear_pages() implementation that hands >PAGE_SIZE
units down to the CPUs.

The result is ~100 lines of code that will compile and run functionally
on any architecture.

To me, that's deserving of an ARCH_HAS_FOO bit that we can set on the
x86 side that then cajoles the core mm/ code to use the fancy new
clear_pages_resched() implementation.

Because what are the arm64 guys going to do when their CPUs start doing
this? They're either going to copy-and-paste the x86 implementation or
they're going to go move the refactor the x86 implementation into common
code.

My money is on the refactoring, because those arm64 guys do good work.
Could we save them the trouble, please?

Oh, and one other little thing:

> +/*
> + * Limit the optimized version of folio_zero_user() to !CONFIG_HIGHMEM.
> + * We do that because clear_pages() works on contiguous kernel pages
> + * which might not be true under HIGHMEM.
> + */

The tip trees are picky about imperative voice, so no "we's". But if you
stick this in mm/, folks are less picky. ;)