[PATCH v6 08/14] mm: bail out of lazy_mmu_mode_* in interrupt context

Kevin Brodsky
2025-12-15T15:04:22Z

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.

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().
Also remove the arm64 handling that is now redundant.

Both arm64 and x86/Xen also ensure that any lazy MMU optimisation is
disabled while in interrupt (see queue_pte_barriers() and
xen_get_lazy_mode() respectively). This will be handled in the
generic layer in a subsequent patch.

Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
arch/arm64/include/asm/pgtable.h | 9 ---------
include/linux/pgtable.h | 17 ++++++++++++++++-
2 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index f7d66c261347..bf9178902bdb 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -94,26 +94,17 @@ static inline void arch_enter_lazy_mmu_mode(void)
* keeps tracking simple.
*/

- if (in_interrupt())
- return;
-
set_thread_flag(TIF_LAZY_MMU);
}

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();
clear_thread_flag(TIF_LAZY_MMU);
}
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 116a18b7916c..dddde6873d1e 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -233,26 +233,41 @@ static inline int pmd_dirty(pmd_t pmd)
* preemption, as a consequence generic code may not sleep while the lazy MMU
* mode is active.
*
- * Nesting is not permitted and the mode cannot be used in interrupt context.
+ * The mode is disabled in interrupt context and calls to the lazy_mmu API have
+ * no effect.
+ *
+ * Nesting is not permitted.
*/
#ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE
static inline void lazy_mmu_mode_enable(void)
{
+ if (in_interrupt())
+ return;
+
arch_enter_lazy_mmu_mode();
}

static inline void lazy_mmu_mode_disable(void)
{
+ if (in_interrupt())
+ return;
+
arch_leave_lazy_mmu_mode();
}

static inline void lazy_mmu_mode_pause(void)
{
+ if (in_interrupt())
+ return;
+
arch_leave_lazy_mmu_mode();
}

static inline void lazy_mmu_mode_resume(void)
{
+ if (in_interrupt())
+ return;
+
arch_enter_lazy_mmu_mode();
}
#else
--
2.51.2