[PATCH 2/4] ARM: Disable HIGHPTE on PREEMPT_RT kernels

[Arnd Bergmann]

From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>

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.

Since HIGHPTE is rarely needed at all, turn it off for PREEMPT_RT
to allow the use of get_user_pages_fast().

[arnd: rework patch to turn off HIGHPTE instead of HAVE_PAST_GUP]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
There is an open question about whether HIGHPTE is still needed
at all, given how rare 32-bit machines with more than 4GB
are on any architecture. If we instead decide to remove HIGHPTE
altogether, this patch is no longer needed.
---
 arch/arm/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index ed850cc0ed3c..4de4e5697bdf 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1231,7 +1231,7 @@ config HIGHMEM
 
 config HIGHPTE
 	bool "Allocate 2nd-level pagetables from highmem" if EXPERT
-	depends on HIGHMEM
+	depends on HIGHMEM && !PREEMPT_RT
 	default y
 	help
 	  The VM uses one page of physical memory for each page table.
-- 
2.39.5