[PATCH v6 09/14] mm: enable lazy_mmu sections to nest

Kevin Brodsky
2025-12-15T15:04:28Z

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 call to leave() disables the batching optimisation before
the outer section ends.

This patch actually enables lazy_mmu sections to nest by tracking
the nesting level in task_struct, in a similar fashion to e.g.
pagefault_{enable,disable}(). This is fully handled by the generic
lazy_mmu helpers that were recently introduced.

lazy_mmu sections were not initially intended to nest, so we need to
clarify the semantics w.r.t. the arch_*_lazy_mmu_mode() callbacks.
This patch takes the following approach:

* The outermost calls to lazy_mmu_mode_{enable,disable}() trigger
calls to arch_{enter,leave}_lazy_mmu_mode() - this is unchanged.

* Nested calls to lazy_mmu_mode_{enable,disable}() are not forwarded
to the arch via arch_{enter,leave} - lazy MMU remains enabled so
the assumption is that these callbacks are not relevant. However,
existing code may rely on a call to disable() to flush any batched
state, regardless of nesting. arch_flush_lazy_mmu_mode() is
therefore called in that situation.

A separate interface was recently introduced to temporarily pause
the lazy MMU mode: lazy_mmu_mode_{pause,resume}(). pause() fully
exits the mode *regardless of the nesting level*, and resume()
restores the mode at the same nesting level.

pause()/resume() are themselves allowed to nest, so we actually
store two nesting levels in task_struct: enable_count and
pause_count. A new helper is_lazy_mmu_mode_active() is introduced to
determine whether we are currently in lazy MMU mode; this will be
used in subsequent patches to replace the various ways arch's
currently track whether the mode is enabled.

In summary (enable/pause represent the values *after* the call):

lazy_mmu_mode_enable() -> arch_enter() enable=1 pause=0
lazy_mmu_mode_enable() -> ø enable=2 pause=0
lazy_mmu_mode_pause() -> arch_leave() enable=2 pause=1
lazy_mmu_mode_resume() -> arch_enter() enable=2 pause=0
lazy_mmu_mode_disable() -> arch_flush() enable=1 pause=0
lazy_mmu_mode_disable() -> arch_leave() enable=0 pause=0

Note: is_lazy_mmu_mode_active() is added to <linux/sched.h> to allow
arch headers included by <linux/pgtable.h> to use it.

Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
arch/arm64/include/asm/pgtable.h | 12 ----
include/linux/mm_types_task.h | 5 ++
include/linux/pgtable.h | 114 +++++++++++++++++++++++++++++--
include/linux/sched.h | 45 ++++++++++++
4 files changed, 157 insertions(+), 19 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index bf9178902bdb..7f528c36d53c 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -82,18 +82,6 @@ static inline void queue_pte_barriers(void)

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.
- */
-
set_thread_flag(TIF_LAZY_MMU);
}

diff --git a/include/linux/mm_types_task.h b/include/linux/mm_types_task.h
index a82aa80c0ba4..11bf319d78ec 100644
--- a/include/linux/mm_types_task.h
+++ b/include/linux/mm_types_task.h
@@ -88,4 +88,9 @@ struct tlbflush_unmap_batch {
#endif
};

+struct lazy_mmu_state {
+ u8 enable_count;
+ u8 pause_count;
+};
+
#endif /* _LINUX_MM_TYPES_TASK_H */
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index dddde6873d1e..2f0dd3a4ace1 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -236,39 +236,139 @@ static inline int pmd_dirty(pmd_t pmd)
* The mode is disabled in interrupt context and calls to the lazy_mmu API have
* no effect.
*
- * Nesting is not permitted.
+ * The lazy MMU mode is enabled for a given block of code using:
+ *
+ * lazy_mmu_mode_enable();
+ *
+ * lazy_mmu_mode_disable();
+ *
+ * Nesting is permitted: may itself use an enable()/disable() pair.
+ * A nested call to enable() has no functional effect; however disable() causes
+ * any batched architectural state to be flushed regardless of nesting. After a
+ * call to disable(), the caller can therefore rely on all previous page table
+ * modifications to have taken effect, but the lazy MMU mode may still be
+ * enabled.
+ *
+ * In certain cases, it may be desirable to temporarily pause the lazy MMU mode.
+ * This can be done using:
+ *
+ * lazy_mmu_mode_pause();
+ *
+ * lazy_mmu_mode_resume();
+ *
+ * pause() ensures that the mode is exited regardless of the nesting level;
+ * resume() re-enters the mode at the same nesting level. Any call to the
+ * lazy_mmu_mode_* API between those two calls has no effect. In particular,
+ * this means that pause()/resume() pairs may nest.
+ *
+ * is_lazy_mmu_mode_active() can be used to check whether the lazy MMU mode is
+ * currently enabled.
*/
#ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
+/**
+ * lazy_mmu_mode_enable() - Enable the lazy MMU mode.
+ *
+ * Enters a new lazy MMU mode section; if the mode was not already enabled,
+ * enables it and calls arch_enter_lazy_mmu_mode().
+ *
+ * Must be paired with a call to lazy_mmu_mode_disable().
+ *
+ * Has no effect if called:
+ * - While paused - see lazy_mmu_mode_pause()
+ * - In interrupt context
+ */
static inline void lazy_mmu_mode_enable(void)
{
- if (in_interrupt())
+ struct lazy_mmu_state *state = &current->lazy_mmu_state;
+
+ if (in_interrupt() || state->pause_count > 0)
return;

- arch_enter_lazy_mmu_mode();
+ VM_WARN_ON_ONCE(state->enable_count == U8_MAX);
+
+ if (state->enable_count++ == 0)
+ arch_enter_lazy_mmu_mode();
}

+/**
+ * lazy_mmu_mode_disable() - Disable the lazy MMU mode.
+ *
+ * Exits the current lazy MMU mode section. If it is the outermost section,
+ * disables the mode and calls arch_leave_lazy_mmu_mode(). Otherwise (nested
+ * section), calls arch_flush_lazy_mmu_mode().
+ *
+ * Must match a call to lazy_mmu_mode_enable().
+ *
+ * Has no effect if called:
+ * - While paused - see lazy_mmu_mode_pause()
+ * - In interrupt context
+ */
static inline void lazy_mmu_mode_disable(void)
{
- if (in_interrupt())
+ struct lazy_mmu_state *state = &current->lazy_mmu_state;
+
+ if (in_interrupt() || state->pause_count > 0)
return;

- arch_leave_lazy_mmu_mode();
+ VM_WARN_ON_ONCE(state->enable_count == 0);
+
+ if (--state->enable_count == 0)
+ arch_leave_lazy_mmu_mode();
+ else /* Exiting a nested section */
+ arch_flush_lazy_mmu_mode();
+
}

+/**
+ * lazy_mmu_mode_pause() - Pause the lazy MMU mode.
+ *
+ * Pauses the lazy MMU mode; if it is currently active, disables it and calls
+ * arch_leave_lazy_mmu_mode().
+ *
+ * Must be paired with a call to lazy_mmu_mode_resume(). Calls to the
+ * lazy_mmu_mode_* API have no effect until the matching resume() call.
+ *
+ * Has no effect if called:
+ * - While paused (inside another pause()/resume() pair)
+ * - In interrupt context
+ */
static inline void lazy_mmu_mode_pause(void)
{
+ struct lazy_mmu_state *state = &current->lazy_mmu_state;
+
if (in_interrupt())
return;

- arch_leave_lazy_mmu_mode();
+ VM_WARN_ON_ONCE(state->pause_count == U8_MAX);
+
+ if (state->pause_count++ == 0 && state->enable_count > 0)
+ arch_leave_lazy_mmu_mode();
}

+/**
+ * lazy_mmu_mode_resume() - Resume the lazy MMU mode.
+ *
+ * Resumes the lazy MMU mode; if it was active at the point where the matching
+ * call to lazy_mmu_mode_pause() was made, re-enables it and calls
+ * arch_enter_lazy_mmu_mode().
+ *
+ * Must match a call to lazy_mmu_mode_pause().
+ *
+ * Has no effect if called:
+ * - While paused (inside another pause()/resume() pair)
+ * - In interrupt context
+ */
static inline void lazy_mmu_mode_resume(void)
{
+ struct lazy_mmu_state *state = &current->lazy_mmu_state;
+
if (in_interrupt())
return;

- arch_enter_lazy_mmu_mode();
+ VM_WARN_ON_ONCE(state->pause_count == 0);
+
+ if (--state->pause_count == 0 && state->enable_count > 0)
+ arch_enter_lazy_mmu_mode();
}
#else
static inline void lazy_mmu_mode_enable(void) {}
diff --git a/include/linux/sched.h b/include/linux/sched.h
index d395f2810fac..f2fc6584c790 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1419,6 +1419,10 @@ struct task_struct {

struct page_frag task_frag;

+#ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
+ struct lazy_mmu_state lazy_mmu_state;
+#endif
+
#ifdef CONFIG_TASK_DELAY_ACCT
struct task_delay_info *delays;
#endif
@@ -1702,6 +1706,47 @@ static inline char task_state_to_char(struct task_struct *tsk)
return task_index_to_char(task_state_index(tsk));
}

+#ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
+/**
+ * __task_lazy_mmu_mode_active() - Test the lazy MMU mode state for a task.
+ * @tsk: The task to check.
+ *
+ * Test whether @tsk has its lazy MMU mode state set to active (i.e. enabled
+ * and not paused).
+ *
+ * This function only considers the state saved in task_struct; to test whether
+ * current actually is in lazy MMU mode, is_lazy_mmu_mode_active() should be
+ * used instead.
+ *
+ * This function is intended for architectures that implement the lazy MMU
+ * mode; it must not be called from generic code.
+ */
+static inline bool __task_lazy_mmu_mode_active(struct task_struct *tsk)
+{
+ struct lazy_mmu_state *state = &tsk->lazy_mmu_state;
+
+ return state->enable_count > 0 && state->pause_count == 0;
+}
+
+/**
+ * is_lazy_mmu_mode_active() - Test whether we are currently in lazy MMU mode.
+ *
+ * Test whether the current context is in lazy MMU mode. This is true if both:
+ * 1. We are not in interrupt context
+ * 2. Lazy MMU mode is active for the current task
+ *
+ * This function is intended for architectures that implement the lazy MMU
+ * mode; it must not be called from generic code.
+ */
+static inline bool is_lazy_mmu_mode_active(void)
+{
+ if (in_interrupt())
+ return false;
+
+ return __task_lazy_mmu_mode_active(current);
+}
+#endif
+
extern struct pid *cad_pid;

/*
--
2.51.2