[PATCH 1/3] mm: Allow pagewalk without locks

[Dev Jain]

It is noted at [1] that KFENCE can manipulate kernel pgtable entries during
softirqs. It does this by calling set_memory_valid() -> __change_memory_common().
This being a non-sleepable context, we cannot take the init_mm mmap lock.
Therefore, add PGWALK_NOLOCK to enable walk_page_range_novma() usage without
locks.

[1] https://lore.kernel.org/linux-arm-kernel/89d0ad18-4772-4d8f-ae8a-7c48d26a927e@arm.com/

Signed-off-by: Dev Jain <dev.jain at arm.com>
---
 include/linux/pagewalk.h |  2 ++
 mm/pagewalk.c            | 12 ++++++++----
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
index 9700a29f8afb..9bc8853ed3de 100644
--- a/include/linux/pagewalk.h
+++ b/include/linux/pagewalk.h
@@ -14,6 +14,8 @@ enum page_walk_lock {
 	PGWALK_WRLOCK = 1,
 	/* vma is expected to be already write-locked during the walk */
 	PGWALK_WRLOCK_VERIFY = 2,
+	/* no lock is needed */
+	PGWALK_NOLOCK = 3,
 };
 
 /**
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index e478777c86e1..9657cf4664b2 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -440,6 +440,8 @@ static inline void process_vma_walk_lock(struct vm_area_struct *vma,
 	case PGWALK_RDLOCK:
 		/* PGWALK_RDLOCK is handled by process_mm_walk_lock */
 		break;
+	default:
+		break;
 	}
 #endif
 }
@@ -640,10 +642,12 @@ int walk_page_range_novma(struct mm_struct *mm, unsigned long start,
 	 * specified address range from being freed. The caller should take
 	 * other actions to prevent this race.
 	 */
-	if (mm == &init_mm)
-		mmap_assert_locked(walk.mm);
-	else
-		mmap_assert_write_locked(walk.mm);
+	if (ops->walk_lock != PGWALK_NOLOCK) {
+		if (mm == &init_mm)
+			mmap_assert_locked(walk.mm);
+		else
+			mmap_assert_write_locked(walk.mm);
+	}
 
 	return walk_pgd_range(start, end, &walk);
 }
-- 
2.30.2