[PATCH v3 1/5] mm: introduce vma_start_read_locked{_nested} helpers

[Suren Baghdasaryan <surenb@google.com>]

Introduce helper functions which can be used to read-lock a VMA when
holding mmap_lock for read. Replace direct accesses to vma->vm_lock
with these new helpers.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 include/linux/mm.h | 24 ++++++++++++++++++++++++
 mm/userfaultfd.c   | 22 +++++-----------------
 2 files changed, 29 insertions(+), 17 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index fecd47239fa9..1ba2e480ae63 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -722,6 +722,30 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
 	return true;
 }
 
+/*
+ * Use only while holding mmap read lock which guarantees that locking will not
+ * fail (nobody can concurrently write-lock the vma). vma_start_read() should
+ * not be used in such cases because it might fail due to mm_lock_seq overflow.
+ * This functionality is used to obtain vma read lock and drop the mmap read lock.
+ */
+static inline void vma_start_read_locked_nested(struct vm_area_struct *vma, int subclass)
+{
+	mmap_assert_locked(vma->vm_mm);
+	down_read_nested(&vma->vm_lock->lock, subclass);
+}
+
+/*
+ * Use only while holding mmap read lock which guarantees that locking will not
+ * fail (nobody can concurrently write-lock the vma). vma_start_read() should
+ * not be used in such cases because it might fail due to mm_lock_seq overflow.
+ * This functionality is used to obtain vma read lock and drop the mmap read lock.
+ */
+static inline void vma_start_read_locked(struct vm_area_struct *vma)
+{
+	mmap_assert_locked(vma->vm_mm);
+	down_read(&vma->vm_lock->lock);
+}
+
 static inline void vma_end_read(struct vm_area_struct *vma)
 {
 	rcu_read_lock(); /* keeps vma alive till the end of up_read */
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 60a0be33766f..87db4b32b82a 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -84,16 +84,8 @@ static struct vm_area_struct *uffd_lock_vma(struct mm_struct *mm,
 
 	mmap_read_lock(mm);
 	vma = find_vma_and_prepare_anon(mm, address);
-	if (!IS_ERR(vma)) {
-		/*
-		 * We cannot use vma_start_read() as it may fail due to
-		 * false locked (see comment in vma_start_read()). We
-		 * can avoid that by directly locking vm_lock under
-		 * mmap_lock, which guarantees that nobody can lock the
-		 * vma for write (vma_start_write()) under us.
-		 */
-		down_read(&vma->vm_lock->lock);
-	}
+	if (!IS_ERR(vma))
+		vma_start_read_locked(vma);
 
 	mmap_read_unlock(mm);
 	return vma;
@@ -1476,14 +1468,10 @@ static int uffd_move_lock(struct mm_struct *mm,
 	mmap_read_lock(mm);
 	err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
 	if (!err) {
-		/*
-		 * See comment in uffd_lock_vma() as to why not using
-		 * vma_start_read() here.
-		 */
-		down_read(&(*dst_vmap)->vm_lock->lock);
+		vma_start_read_locked(*dst_vmap);
 		if (*dst_vmap != *src_vmap)
-			down_read_nested(&(*src_vmap)->vm_lock->lock,
-					 SINGLE_DEPTH_NESTING);
+			vma_start_read_locked_nested(*src_vmap,
+						SINGLE_DEPTH_NESTING);
 	}
 	mmap_read_unlock(mm);
 	return err;
-- 
2.47.0.338.g60cca15819-goog