linux-kernel - Re: [PATCH v8 5/7] x86/clear_page: Introduce clear

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Message-ID: <20251028135640.GBaQDLmHzCQDegBHd6@fat_crate.local>
Date: Tue, 28 Oct 2025 14:56:40 +0100
From: Borislav Petkov <bp@...en8.de>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
	akpm@...ux-foundation.org, david@...hat.com,
	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 v8 5/7] x86/clear_page: Introduce clear_pages()

On Mon, Oct 27, 2025 at 01:21:07PM -0700, Ankur Arora wrote:
> Also define ARCH_PAGE_CONTIG_NR to specify the maximum contiguous page
> range that can be zeroed when running under cooperative preemption
> models. This limits the worst case preemption latency.

Please do not explain what the patch does in the commit message - that should
be clear from the diff itself. Rather, concentrate on why this patch exists.

> +/*
> + * When running under cooperatively scheduled preemption models limit the
> + * maximum contiguous extent that can be cleared to pages worth 8MB.

Why?

> + *
> + * With a clearing BW of ~10GBps, this should result in worst case scheduling

This sounds like you have this bandwidth (please write it out - we have more
than enough silly abbreviations) on *everything* x86 the kernel runs on. Which
simply ain't true.

> + * latency of ~1ms.
> + */
> +#define ARCH_PAGE_CONTIG_NR (8 << (20 - PAGE_SHIFT))

And so this looks like some magic number which makes sense only on some
uarches but most likely it doesn't on others.

Why isn't this thing determined dynamically during boot or so, instead of
hardcoding it this way and then having to change it again later when bandwidth
increases?

Hmm, weird.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette