Subject: [PATCH] arm: Disable FAST_GUP on PREEMPT_RT if HIGHPTE is also enabled.
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Tue, 23 Jan 2024 12:56:21 +0100

gup_pgd_range() is invoked with disabled interrupts and invokes
__kmap_local_page_prot() via pte_offset_map(), gup_p4d_range().
With HIGHPTE enabled, __kmap_local_page_prot() invokes kmap_high_get()
which uses a spinlock_t via lock_kmap_any(). This leads to an
sleeping-while-atomic error on PREEMPT_RT because spinlock_t becomes a
sleeping lock and must not be acquired in atomic context.

The loop in map_new_virtual() uses wait_queue_head_t for wake up which
also is using a spinlock_t.

Limit HAVE_FAST_GUP additionaly to remain disabled on PREEMPT_RT with
HIGHPTE enabled.

Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 arch/arm/Kconfig |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -103,7 +103,7 @@ config ARM
 	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
 	select HAVE_EXIT_THREAD
-	select HAVE_GUP_FAST if ARM_LPAE
+	select HAVE_GUP_FAST if ARM_LPAE && !(PREEMPT_RT && HIGHPTE)
 	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
 	select HAVE_FUNCTION_ERROR_INJECTION
 	select HAVE_FUNCTION_GRAPH_TRACER