[PATCH 3/3] arm64: mm: Make arch_faults_on_old_pte() check for migratability

[Sebastian Andrzej Siewior]

From: Valentin Schneider <valentin.schneider at arm.com>

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().

Cc: Valentin Schneider <vschneid at redhat.com>
Suggested-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
Signed-off-by: Valentin Schneider <valentin.schneider at arm.com>
Link: https://lore.kernel.org/r/20220127192437.1192957-1-valentin.schneider@arm.com
Acked-by: Catalin Marinas <catalin.marinas at arm.com>
---
 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 8ebf1cec5d901..5a53558578ab3 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -1018,7 +1018,8 @@ 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();
 }
-- 
2.36.0