linux-kernel - Re: [PATCH 2/4] ARM: Disable HIGHPTE on PREEMPT

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Message-ID: <CACRpkdbYU=Txm0zUWDWvNXA0JHRNGpy1ccy0zQdiBb2Ya+UBFA@mail.gmail.com>
Date: Fri, 13 Dec 2024 01:27:00 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Arnd Bergmann <arnd@...nel.org>, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, linux-mm@...ck.org, 
	linux-rt-devel@...ts.linux.dev, Ard Biesheuvel <ardb@...nel.org>, 
	Clark Williams <clrkwllms@...nel.org>, Jason Baron <jbaron@...mai.com>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Matthew Wilcox <willy@...radead.org>, Peter Zijlstra <peterz@...radead.org>, 
	Russell King <linux@...linux.org.uk>, Steven Rostedt <rostedt@...dmis.org>, 
	David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 2/4] ARM: Disable HIGHPTE on PREEMPT_RT kernels

On Wed, Dec 11, 2024 at 4:22 PM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
> On 2024-12-11 14:29:29 [+0100], Linus Walleij wrote:
> > So fast GUP is supposed to be lockless, and should just not
> > have this problem. So it can't be addressing gup_fast_pgd_range()
> > right?
> …
> > I'm more asking if HIGHPTE even acquires a spinlock anymore
> > as it is supposed to be "fast"/lockless. If it does, it is clearly violating
> > the "fast" promise of the fast GUP API and should not exist.
>
> This is lockless on x86. The problem is ARM's
> arch_kmap_local_high_get(). This is where the lock is from.

Aha that calls down to kmap_high_get() that that issues
lock_kmap_any(flags).

But is it really sound that the "fast" API does this? It feels
like a violation of the whole design of the fast stuff.

Yours,
Linus Walleij