[PATCH v2 3/7] arm64: mm: fully support nested lazy_mmu sections

Kevin Brodsky
2025-09-08T07:40:49Z

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@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