linux-kernel - Re: [PATCH v4] arm64: mm: Make arch_faults_on_old

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Date:   Tue, 15 Feb 2022 15:37:27 +0000
From:   Will Deacon <will@...nel.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-rt-users@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Steven Price <steven.price@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH v4] arm64: mm: Make arch_faults_on_old_pte() check for
 migratability

On Thu, Jan 27, 2022 at 07:24:37PM +0000, Valentin Schneider wrote:
> arch_faults_on_old_pte() relies on the calling context being
> non-preemptible. CONFIG_PREEMPT_RT turns the PTE lock into a sleepable
> spinlock, which doesn't disable preemption once acquired, triggering the
> warning in arch_faults_on_old_pte().
> 
> It does however disable migration, ensuring the task remains on the same
> CPU during the entirety of the critical section, making the read of
> cpu_has_hw_af() safe and stable.
> 
> Make arch_faults_on_old_pte() check cant_migrate() instead of preemptible().
> 
> Suggested-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> Link: https://lore.kernel.org/r/20210811201354.1976839-5-valentin.schneider@arm.com
> ---
> v3 -> v4: Dropped migratable(), reuse cant_migrate() (Sebastian)
> ---
>  arch/arm64/include/asm/pgtable.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index c4ba047a82d2..3caf6346ea95 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -1001,7 +1001,8 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
>   */
>  static inline bool arch_faults_on_old_pte(void)
>  {
> -	WARN_ON(preemptible());
> +	/* The register read below requires a stable CPU to make any sense */
> +	cant_migrate();

FWIW: There's a patch in the multi-generational LRU series that drops this
WARN_ON() entirely. I'm not sure if/when that lot will land, but just wanted
to let you know.

Will