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

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Message-ID: <Z1v6XcaSK97Sa9Tz@shell.armlinux.org.uk>
Date: Fri, 13 Dec 2024 09:11:57 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Linus Walleij <linus.walleij@...aro.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	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>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 2/4] ARM: Disable HIGHPTE on PREEMPT_RT kernels

On Fri, Dec 13, 2024 at 01:27:00AM +0100, Linus Walleij wrote:
> 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.

If there's no way to do it lockless, then it has to take the lock.

-- 
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