[PATCH 3/7] arm64: mm: fully support nested lazy_mmu sections

[Kevin Brodsky]

Despite recent efforts to prevent lazy_mmu sections from nesting, it
remains difficult to ensure that it never occurs - and in fact it
does occur on arm64 in certain situations (CONFIG_DEBUG_PAGEALLOC).
Commit 1ef3095b1405 ("arm64/mm: Permit lazy_mmu_mode to be nested")
made nesting tolerable on arm64, but without truly supporting it:
the inner leave() call clears TIF_LAZY_MMU, disabling the batching
optimisation before the outer section ends.

Now that the lazy_mmu API allows enter() to pass through a state to
the matching leave() call, we can actually support nesting. If
enter() is called inside an active lazy_mmu section, TIF_LAZY_MMU
will already be set, and we can then return LAZY_MMU_NESTED to
instruct the matching leave() call not to clear TIF_LAZY_MMU.

The only effect of this patch is to ensure that TIF_LAZY_MMU (and
therefore the batching optimisation) remains set until the outermost
lazy_mmu section ends. leave() still emits barriers if needed,
regardless of the nesting level, as the caller may expect any
page table changes to become visible when leave() returns.

Signed-off-by: Kevin Brodsky <kevin.brodsky at arm.com>
---
 arch/arm64/include/asm/pgtable.h | 19 +++++--------------
 1 file changed, 5 insertions(+), 14 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 816197d08165..602feda97dc4 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -85,24 +85,14 @@ typedef int lazy_mmu_state_t;
 
 static inline lazy_mmu_state_t 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.
-	 */
+	int lazy_mmu_nested;
 
 	if (in_interrupt())
 		return LAZY_MMU_DEFAULT;
 
-	set_thread_flag(TIF_LAZY_MMU);
+	lazy_mmu_nested = test_and_set_thread_flag(TIF_LAZY_MMU);
 
-	return LAZY_MMU_DEFAULT;
+	return lazy_mmu_nested ? LAZY_MMU_NESTED : LAZY_MMU_DEFAULT;
 }
 
 static inline void arch_leave_lazy_mmu_mode(lazy_mmu_state_t state)
@@ -113,7 +103,8 @@ static inline void arch_leave_lazy_mmu_mode(lazy_mmu_state_t state)
 	if (test_and_clear_thread_flag(TIF_LAZY_MMU_PENDING))
 		emit_pte_barriers();
 
-	clear_thread_flag(TIF_LAZY_MMU);
+	if (state != LAZY_MMU_NESTED)
+		clear_thread_flag(TIF_LAZY_MMU);
 }
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
-- 
2.47.0