[PATCH 20/30] mm/vma: introduce vma_assert_can_modify()

vma_assert_write_locked() and vma_assert_attached() are useful for their
own purposes, however VMA code absolutely does allow the modification of
non-write locked VMAs if they are at that point detached (i.e. unreachable
from anywhere).

It's therefore useful to be able to assert that a VMA is either
detached (modification doesn't matter) or write locked (you're explicitly
locked for modification).

Therefore introduce vma_assert_can_modify() for this purpose.

While we're here, make vma_is_attached() available generally - if
!CONFIG_PER_VMA_LOCKS, then there's no sense in which a VMA is
detached (vma_mark_detached() is a noop), so have this default to true in
this case.

Signed-off-by: Lorenzo Stoakes <[email protected]>
---
 include/linux/mmap_lock.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 04b8f61ece5d..d513286d8160 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -506,6 +506,8 @@ static inline __must_check
 int vma_start_write_killable(struct vm_area_struct *vma) { return 0; }
 static inline void vma_assert_write_locked(struct vm_area_struct *vma)
 		{ mmap_assert_write_locked(vma->vm_mm); }
+static inline bool vma_is_attached(struct vm_area_struct *vma)
+		{ return true; }
 static inline void vma_assert_attached(struct vm_area_struct *vma) {}
 static inline void vma_assert_detached(struct vm_area_struct *vma) {}
 static inline void vma_mark_attached(struct vm_area_struct *vma) {}
@@ -530,6 +532,12 @@ static inline void vma_assert_stabilised(struct vm_area_struct *vma)
 
 #endif /* CONFIG_PER_VMA_LOCK */
 
+static inline void vma_assert_can_modify(struct vm_area_struct *vma)
+{
+	if (vma_is_attached(vma))
+		vma_assert_write_locked(vma);
+}
+
 static inline void mmap_write_lock(struct mm_struct *mm)
 {
 	__mmap_lock_trace_start_locking(mm, true);
-- 
2.54.0