[PATCH v4 12/12] mm: bail out of lazy_mmu_mode_* in interrupt context

[Kevin Brodsky]

The lazy MMU mode cannot be used in interrupt context. This is
documented in <linux/pgtable.h>, but isn't consistently handled
across architectures.

arm64 ensures that calls to lazy_mmu_mode_* have no effect in
interrupt context, because such calls do occur in certain
configurations - see commit b81c688426a9 ("arm64/mm: Disable barrier
batching in interrupt contexts"). Other architectures do not check
this situation, most likely because it hasn't occurred so far.

Both arm64 and x86/Xen also ensure that any lazy MMU optimisation is
disabled while in interrupt mode (see queue_pte_barriers() and
xen_get_lazy_mode() respectively).

Let's handle this in the new generic lazy_mmu layer, in the same
fashion as arm64: bail out of lazy_mmu_mode_* if in_interrupt(), and
have in_lazy_mmu_mode() return false to disable any optimisation.
Also remove the arm64 handling that is now redundant; x86/Xen has
its own internal tracking so it is left unchanged.

Signed-off-by: Kevin Brodsky <kevin.brodsky at arm.com>
---
 arch/arm64/include/asm/pgtable.h | 17 +----------------
 include/linux/pgtable.h          | 16 ++++++++++++++--
 include/linux/sched.h            |  3 +++
 3 files changed, 18 insertions(+), 18 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 61ca88f94551..96987a49e83b 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -62,37 +62,22 @@ static inline void emit_pte_barriers(void)
 
 static inline void queue_pte_barriers(void)
 {
-	if (in_interrupt()) {
-		emit_pte_barriers();
-		return;
-	}
-
 	if (in_lazy_mmu_mode())
 		test_and_set_thread_flag(TIF_LAZY_MMU_PENDING);
 	else
 		emit_pte_barriers();
 }
 
-static inline void arch_enter_lazy_mmu_mode(void)
-{
-	if (in_interrupt())
-		return;
-}
+static inline void arch_enter_lazy_mmu_mode(void) {}
 
 static inline void arch_flush_lazy_mmu_mode(void)
 {
-	if (in_interrupt())
-		return;
-
 	if (test_and_clear_thread_flag(TIF_LAZY_MMU_PENDING))
 		emit_pte_barriers();
 }
 
 static inline void arch_leave_lazy_mmu_mode(void)
 {
-	if (in_interrupt())
-		return;
-
 	arch_flush_lazy_mmu_mode();
 }
 
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index e6064e00b22d..e6069ce4ec83 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -228,8 +228,8 @@ static inline int pmd_dirty(pmd_t pmd)
  * of the lazy mode. So the implementation must assume preemption may be enabled
  * and cpu migration is possible; it must take steps to be robust against this.
  * (In practice, for user PTE updates, the appropriate page table lock(s) are
- * held, but for kernel PTE updates, no lock is held). The mode cannot be used
- * in interrupt context.
+ * held, but for kernel PTE updates, no lock is held). The mode is disabled
+ * in interrupt context and calls to the lazy_mmu API have no effect.
  *
  * The lazy MMU mode is enabled for a given block of code using:
  *
@@ -265,6 +265,9 @@ static inline void lazy_mmu_mode_enable(void)
 {
 	struct lazy_mmu_state *state = &current->lazy_mmu_state;
 
+	if (in_interrupt())
+		return;
+
 	VM_WARN_ON_ONCE(state->nesting_level == U8_MAX);
 	/* enable() must not be called while paused */
 	VM_WARN_ON(state->nesting_level > 0 && !state->active);
@@ -279,6 +282,9 @@ static inline void lazy_mmu_mode_disable(void)
 {
 	struct lazy_mmu_state *state = &current->lazy_mmu_state;
 
+	if (in_interrupt())
+		return;
+
 	VM_WARN_ON_ONCE(state->nesting_level == 0);
 	VM_WARN_ON(!state->active);
 
@@ -295,6 +301,9 @@ static inline void lazy_mmu_mode_pause(void)
 {
 	struct lazy_mmu_state *state = &current->lazy_mmu_state;
 
+	if (in_interrupt())
+		return;
+
 	VM_WARN_ON(state->nesting_level == 0 || !state->active);
 
 	state->active = false;
@@ -305,6 +314,9 @@ static inline void lazy_mmu_mode_resume(void)
 {
 	struct lazy_mmu_state *state = &current->lazy_mmu_state;
 
+	if (in_interrupt())
+		return;
+
 	VM_WARN_ON(state->nesting_level == 0 || state->active);
 
 	state->active = true;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 11566d973f42..bb873016ffcf 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1731,6 +1731,9 @@ static inline char task_state_to_char(struct task_struct *tsk)
 #ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
 static inline bool in_lazy_mmu_mode(void)
 {
+	if (in_interrupt())
+		return false;
+
 	return current->lazy_mmu_state.active;
 }
 #else
-- 
2.47.0