[PATCH v6 06/16] mm: allow vma_start_read_locked/vma_start_read_locked_nested to fail

[Suren Baghdasaryan <surenb@google.com>]

With upcoming replacement of vm_lock with vm_refcnt, we need to handle a
possibility of vma_start_read_locked/vma_start_read_locked_nested failing
due to refcount overflow. Prepare for such possibility by changing these
APIs and adjusting their users.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
---
 include/linux/mm.h |  6 ++++--
 mm/userfaultfd.c   | 17 ++++++++++++-----
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 689f5a1e2181..0ecd321c50b7 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -747,10 +747,11 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
  * 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)
+static inline bool 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);
+	return true;
 }
 
 /*
@@ -759,10 +760,11 @@ static inline void vma_start_read_locked_nested(struct vm_area_struct *vma, int
  * 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)
+static inline bool vma_start_read_locked(struct vm_area_struct *vma)
 {
 	mmap_assert_locked(vma->vm_mm);
 	down_read(&vma->vm_lock.lock);
+	return true;
 }
 
 static inline void vma_end_read(struct vm_area_struct *vma)
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index bc9a66ec6a6e..79e8ae676f75 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -85,7 +85,8 @@ 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))
-		vma_start_read_locked(vma);
+		if (!vma_start_read_locked(vma))
+			vma = ERR_PTR(-EAGAIN);
 
 	mmap_read_unlock(mm);
 	return vma;
@@ -1483,10 +1484,16 @@ 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) {
-		vma_start_read_locked(*dst_vmap);
-		if (*dst_vmap != *src_vmap)
-			vma_start_read_locked_nested(*src_vmap,
-						SINGLE_DEPTH_NESTING);
+		if (!vma_start_read_locked(*dst_vmap)) {
+			if (*dst_vmap != *src_vmap) {
+				if (!vma_start_read_locked_nested(*src_vmap,
+							SINGLE_DEPTH_NESTING)) {
+					vma_end_read(*dst_vmap);
+					err = -EAGAIN;
+				}
+			}
+		} else
+			err = -EAGAIN;
 	}
 	mmap_read_unlock(mm);
 	return err;
-- 
2.47.1.613.gc27f4b7a9f-goog