[PATCH v7 01/17] mm: introduce vma_start_read_locked{_nested} helpers

Suren Baghdasaryan
2024-12-26T17:07:19Z

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>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
---
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 406b981af881..a48e207d25f2 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -735,6 +735,30 @@ 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 af3dfc3633db..4527c385935b 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;
@@ -1491,14 +1483,10 @@ 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;
--
2.47.1.613.gc27f4b7a9f-goog