linux-kernel - Re: [PATCH v3 00/21] huge page clearing optimizations

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Date:   Wed, 8 Jun 2022 12:39:45 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Ankur Arora <ankur.a.arora@...cle.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <ak@...ux.intel.com>, Arnd Bergmann <arnd@...db.de>,
        Jason Gunthorpe <jgg@...dia.com>, jon.grimm@....com,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Joao Martins <joao.m.martins@...cle.com>
Subject: Re: [PATCH v3 00/21] huge page clearing optimizations

On Wed, Jun 8, 2022 at 12:25 PM Ankur Arora <ankur.a.arora@...cle.com> wrote:
>
> But, even on x86, AFAICT gigantic pages could straddle MAX_SECTION_BITS?
> An arch specific clear_huge_page() code could, however handle 1GB pages
> via some kind of static loop around (30 - MAX_SECTION_BITS).

Even if gigantic pages straddle that area, it simply shouldn't matter.

The only reason that MAX_SECTION_BITS matters is for the 'struct page *' lookup.

And the only reason for *that* is because of HIGHMEM.

So it's all entirely silly and pointless on any sane architecture, I think.

> We'll need a preemption point there for CONFIG_PREEMPT_VOLUNTARY
> as well, right?

Ahh, yes.  I should have looked at the code, and not just gone by my
"PREEMPT_NONE vs PREEMPT" thing that entirely forgot about how we
split that up.

> Just one minor point -- seems to me that the choice of nontemporal or
> temporal might have to be based on a hint to clear_huge_page().

Quite possibly. But I'd prefer that  as a separate "look, this
improves numbers by X%" thing from the whole "let's make the
clear_huge_page() interface at least sane".

                 Linus