ARM: 9461/1: Disable HIGHPTE on PREEMPT_RT kernels

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]

Co-developed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 92ca24336ec4b99cdafe14bbb79ea48605a1bb79..45d29320196f0d51b278d3687778fe016a0b1ec3 100644 (file)
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1213,7 +1213,7 @@ 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.