[PATCH v7 08/17] mm: allow vma_start_read_locked/vma_start_read_locked_nested to fail

Suren Baghdasaryan
2024-12-26T17:07:36Z

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 c50edfedd99d..ab27de9729d8 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 4527c385935b..38207d8be205 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