linux-kernel - Re: [PATCH] RFC: ARM: drop support for HIGHPTE

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Message-ID: <CACRpkdZ=4n=_yeWY1MSDRuQ=H-P=QiTcQkJF_YDHMfRM2RgDZw@mail.gmail.com>
Date: Wed, 10 Sep 2025 14:25:14 +0200
From: Linus Walleij <linus.walleij@...aro.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: Russell King <linux@...linux.org.uk>, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, Russell King <rmk+kernel@...linux.org.uk>, 
	Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] RFC: ARM: drop support for HIGHPTE

On Wed, Sep 10, 2025 at 2:15 PM Arnd Bergmann <arnd@...db.de> wrote:
> On Wed, Sep 10, 2025, at 11:47, Linus Walleij wrote:
> > Matthew Wilcox suggests HIGHPTE is a burden for the
> > kernel MM, and only one ARM system (vt8500) is currently
> > using it.
>
> I think the vt8500_v6_v7_defconfig entry is a historic leftover,
> instead it is currently enabled by default on all configurations
> that enable highmem.
>
> > This deletes the use of HIGHPTE in ARM32 clearing the way
> > to remove the feature altogether.
> >
> > It was introduced in commit 65cec8e3db60 "ARM: implement highpte"
> > in 2009, so this RFC is intended as a discussion item to
> > check if it is still a desired feature 16 years later
> > or if we can get rid of it.
>
> We discussed it last year in the context of the PREEMPT_RT patches, see
> https://lore.kernel.org/linux-arm-kernel/20241210160556.2341497-4-arnd@kernel.org/
> and Russell's reply to that.
>
> I still think that dropping it is the sensible tradeoff here in
> the end, as it should allow further simplifications in the
> pgalloc code. Your version of this patch doesn't (like my
> identical patch from December) actually do any of that though.

Ah, I was blissfully unaware of that patch, sorry.

If we go any further with this let's rebase your patch instead.

Yours,
Linus Walleij