[PATCH v2 4/4] arm64: mm: Make arch_faults_on_old_pte() check for migratability

[Valentin Schneider]

Running v5.13-rt1 on my arm64 Juno board triggers:

[   30.430643] WARNING: CPU: 4 PID: 1 at arch/arm64/include/asm/pgtable.h:985 do_set_pte (./arch/arm64/include/asm/pgtable.h:985 ./arch/arm64/include/asm/pgtable.h:997 mm/memory.c:3830)
[   30.430669] Modules linked in:
[   30.430679] CPU: 4 PID: 1 Comm: init Tainted: G        W         5.13.0-rt1-00002-gcb994ad7c570 #35
[   30.430690] Hardware name: ARM Juno development board (r0) (DT)
[   30.430695] pstate: 80000005 (Nzcv daif -PAN -UAO -TCO BTYPE=--)
[   30.430874] Call trace:
[   30.430878] do_set_pte (./arch/arm64/include/asm/pgtable.h:985 ./arch/arm64/include/asm/pgtable.h:997 mm/memory.c:3830)
[   30.430886] filemap_map_pages (mm/filemap.c:3222)
[   30.430895] __handle_mm_fault (mm/memory.c:4006 mm/memory.c:4020 mm/memory.c:4153 mm/memory.c:4412 mm/memory.c:4547)
[   30.430904] handle_mm_fault (mm/memory.c:4645)
[   30.430912] do_page_fault (arch/arm64/mm/fault.c:507 arch/arm64/mm/fault.c:607)
[   30.430925] do_translation_fault (arch/arm64/mm/fault.c:692)
[   30.430936] do_mem_abort (arch/arm64/mm/fault.c:821)
[   30.430946] el0_ia (arch/arm64/kernel/entry-common.c:324)
[   30.430959] el0_sync_handler (arch/arm64/kernel/entry-common.c:431)
[   30.430967] el0_sync (arch/arm64/kernel/entry.S:744)
[   30.430977] irq event stamp: 1228384
[   30.430981] hardirqs last enabled at (1228383): lock_page_memcg (mm/memcontrol.c:2005 (discriminator 1))
[   30.430993] hardirqs last disabled at (1228384): el1_dbg (arch/arm64/kernel/entry-common.c:144 arch/arm64/kernel/entry-common.c:234)
[   30.431007] softirqs last enabled at (1228260): __local_bh_enable_ip (./arch/arm64/include/asm/irqflags.h:85 kernel/softirq.c:262)
[   30.431022] softirqs last disabled at (1228232): fpsimd_restore_current_state (./include/linux/bottom_half.h:19 arch/arm64/kernel/fpsimd.c:183 arch/arm64/kernel/fpsimd.c:1182)

CONFIG_PREEMPT_RT turns the PTE lock into a sleepable spinlock. Since
acquiring such a lock also disables migration, any per-CPU access done
under the lock remains safe even if preemptible.

This affects:

  filemap_map_pages()
  `\
    do_set_pte()
    `\
      arch_wants_old_prefaulted_pte()

which checks preemptible() to figure out if the output of
cpu_has_hw_af() (IOW the underlying CPU) will remain stable for the
subsequent operations. Make it use is_pcpu_safe() instead.

Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
---
 arch/arm64/include/asm/pgtable.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index f09bf5c02891..767a064bedde 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -995,7 +995,7 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
  */
 static inline bool arch_faults_on_old_pte(void)
 {
-	WARN_ON(preemptible());
+	WARN_ON(!is_pcpu_safe());
 
 	return !cpu_has_hw_af();
 }
-- 
2.25.1