From 9b9c42f04cb670b48cbd17094498dc2b028494b0 Mon Sep 17 00:00:00 2001
From: Valentin Schneider <valentin.schneider@arm.com>
Date: Thu, 27 Jan 2022 19:24:37 +0000
Subject: [PATCH] arm64: mm: Make arch_faults_on_old_pte() check for
 migratability

References: SLE Realtime Extension
Patch-mainline: Queued in subsystem maintainer repository
Git-commit: e34efd635f01a49d1efdd76c8db699072faf610b
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git

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/20220127192437.1192957-1-valentin.schneider@arm.com
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 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 16e53d251508..df6088203e84 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();
 }