[RFC PATCH v1 6/6] Revert "arm64/mm: Permit lazy_mmu_mode to be nested"

[Ryan Roberts <ryan.roberts@arm.com>]

Commit 491344301b25 ("arm64/mm: Permit lazy_mmu_mode to be nested") made
the arm64 implementation of lazy_mmu_mode tolerant to nesting. But
subsequent commits have fixed the core code to ensure that lazy_mmu_mode
never gets nested (as originally intended). Therefore we can revert this
commit and reinstate the VM_WARN() if nesting is detected in future.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 arch/arm64/include/asm/pgtable.h | 14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index add75dee49f5..dcf0adbeb803 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -83,21 +83,11 @@ static inline void queue_pte_barriers(void)
 #define  __HAVE_ARCH_ENTER_LAZY_MMU_MODE
 static inline void arch_enter_lazy_mmu_mode(void)
 {
-	/*
-	 * lazy_mmu_mode is not supposed to permit nesting. But in practice this
-	 * does happen with CONFIG_DEBUG_PAGEALLOC, where a page allocation
-	 * inside a lazy_mmu_mode section (such as zap_pte_range()) will change
-	 * permissions on the linear map with apply_to_page_range(), which
-	 * re-enters lazy_mmu_mode. So we tolerate nesting in our
-	 * implementation. The first call to arch_leave_lazy_mmu_mode() will
-	 * flush and clear the flag such that the remainder of the work in the
-	 * outer nest behaves as if outside of lazy mmu mode. This is safe and
-	 * keeps tracking simple.
-	 */
-
 	if (in_interrupt())
 		return;
 
+	VM_WARN_ON(test_thread_flag(TIF_LAZY_MMU));
+
 	set_thread_flag(TIF_LAZY_MMU);
 }
 
-- 
2.43.0