[v4] arm64: mm: Make arch_faults_on_old_pte() check for migratability

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@linutronix.de>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.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(-)

On 2022-01-27 19:24:37 [+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@linutronix.de>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> Link: https://lore.kernel.org/r/20210811201354.1976839-5-valentin.schneider@arm.com

Let me pick that up so I can drop the other two. I hope the ARM64 folks
follow my lead ;)

Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Sebastian

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@linutronix.de>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.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();
>  
>  	return !cpu_has_hw_af();
>  }

The patch looks alright but what's the plan with it? Does it go into the
preempt-rt tree, mainline? It's not a fix to be queued now, so:

Acked-by: Catalin Marinas <catalin.marinas@arm.com>

On 2022-02-04 18:55:17 [+0000], Catalin Marinas wrote:
> 
> The patch looks alright but what's the plan with it? Does it go into the
> preempt-rt tree, mainline? It's not a fix to be queued now, so:

It would be nice if if it could go mainline, too.

> Acked-by: Catalin Marinas <catalin.marinas@arm.com>

Sebastian

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@linutronix.de>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.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