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

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Date:   Thu, 27 Jan 2022 19:24:37 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-rt-users@...r.kernel.org
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, 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: [PATCH 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@...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();
 
 	return !cpu_has_hw_af();
 }
-- 
2.25.1