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

[Will Deacon]

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@xxxxxxxxxxxxx>
> Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
> Link: https://lore.kernel.org/r/20210811201354.1976839-5-valentin.schneider@xxxxxxx
> ---
> 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