[PATCH v6 05/14] mm: clarify lazy_mmu sleeping constraints

Kevin Brodsky
2025-12-15T15:04:05Z

The lazy MMU mode documentation makes clear that an implementation
should not assume that preemption is disabled or any lock is held
upon entry to the mode; however it says nothing about what code
using the lazy MMU interface should expect.

In practice sleeping is forbidden (for generic code) while the lazy
MMU mode is active: say it explicitly.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
include/linux/pgtable.h | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 652f287c1ef6..1abc4a1c3d72 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -225,11 +225,15 @@ static inline int pmd_dirty(pmd_t pmd)
* up to date.
*
* In the general case, no lock is guaranteed to be held between entry and exit
- * 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). Nesting is not permitted
- * and the mode cannot be used in interrupt context.
+ * of the lazy mode. (In practice, for user PTE updates, the appropriate page
+ * table lock(s) are held, but for kernel PTE updates, no lock is held).
+ * The implementation must therefore assume preemption may be enabled upon
+ * entry to the mode and cpu migration is possible; it must take steps to be
+ * robust against this. An implementation may handle this by disabling
+ * 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.
*/
#ifndef __HAVE_ARCH_ENTER_LAZY_MMU_MODE
static inline void arch_enter_lazy_mmu_mode(void) {}
--
2.51.2