[PATCH 2/3] mm: only assert VMA locks if CONFIG_DEBUG_VM is set

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

Similar to the mmap lock asserts, we have also asserted VMA locks
regardless of whether or not the kernel is a debug build or not.

Similarly, these are asserting basic assumptions about kernel code which
should never assert in production code.

Since all such bugs are generally asserted using VM_WARN_ON_ONCE() or
similar, it creates an odd and inconsistent situation where some
programmatic asserts are only checked under CONFIG_DEBUG_VM and some are
always checked.

Since the intent is the same as any VM_WARN_ON_ONCE() check, do not assert
if CONFIG_DEBUG_VM is not set.

Implement by updating vma_assert_attached() and vma_assert_detached() to
use VM_WARN_ON_ONCE() rather than WARN_ON_ONCE(), and gating
vma_assert_[write_]locked() and vma_assert_can_modify() on CONFIG_DEBUG_VM.

nommu loses its CONFIG_LOCKDEP asserts if !CONFIG_DEBUG_VM in
vma_assert_[write_]locked(), but this doesn't seem to be a problem.

In addition, there were already inconsistencies - vma_assert_write_locked()
uses VM_WARN_ON_ONCE_VMA() and performs no actions if CONFIG_DEBUG_VM is
not set, and vma_assert_locked() looked up vma->vm_refcnt but also only
asserted if CONFIG_DEBUG_VM was set.

Therefore simply update these functions such that they're predicated on
CONFIG_DEBUG_VM and update their kdoc descriptions accordingly.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 include/linux/mmap_lock.h | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 9367430ee53c..bada7a03ddf4 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -354,9 +354,14 @@ int vma_start_write_killable(struct vm_area_struct *vma)
 /**
  * vma_assert_write_locked() - assert that @vma holds a VMA write lock.
  * @vma: The VMA to assert.
+ *
+ * Assert only if CONFIG_DEBUG_VM is set.
  */
 static inline void vma_assert_write_locked(const struct vm_area_struct *vma)
 {
+	if (!IS_ENABLED(CONFIG_DEBUG_VM))
+		return;
+
 	if (!IS_ENABLED(CONFIG_MMU)) {
 		mmap_assert_write_locked(vma->vm_mm);
 		return;
@@ -369,11 +374,16 @@ static inline void vma_assert_write_locked(const struct vm_area_struct *vma)
  * vma_assert_locked() - assert that @vma holds either a VMA read or a VMA write
  * lock and is not detached.
  * @vma: The VMA to assert.
+ *
+ * Assert only if CONFIG_DEBUG_VM is set.
  */
 static inline void vma_assert_locked(const struct vm_area_struct *vma)
 {
 	unsigned int refcnt;
 
+	if (!IS_ENABLED(CONFIG_DEBUG_VM))
+		return;
+
 	if (!IS_ENABLED(CONFIG_MMU)) {
 		mmap_assert_locked(vma->vm_mm);
 		return;
@@ -480,12 +490,12 @@ static inline void vma_assert_attached(struct vm_area_struct *vma)
 	if (!IS_ENABLED(CONFIG_MMU))
 		return;
 
-	WARN_ON_ONCE(!vma_is_attached(vma));
+	VM_WARN_ON_ONCE(!vma_is_attached(vma));
 }
 
 static inline void vma_assert_detached(struct vm_area_struct *vma)
 {
-	WARN_ON_ONCE(vma_is_attached(vma));
+	VM_WARN_ON_ONCE(vma_is_attached(vma));
 }
 
 static inline void vma_mark_attached(struct vm_area_struct *vma)
@@ -549,6 +559,9 @@ struct vm_area_struct *lock_next_vma(struct vma_rcu_iter *rvmi,
 
 static inline void vma_assert_can_modify(struct vm_area_struct *vma)
 {
+	if (!IS_ENABLED(CONFIG_DEBUG_VM))
+		return;
+
 	if (vma_is_attached(vma))
 		vma_assert_write_locked(vma);
 }

-- 
2.56.0