[PATCH v10 16/18] mm: prepare lock_vma_under_rcu() for vma reuse possibility

[Suren Baghdasaryan <surenb@google.com>]

Once we make vma cache SLAB_TYPESAFE_BY_RCU, it will be possible for a vma
to be reused and attached to another mm after lock_vma_under_rcu() locks
the vma. lock_vma_under_rcu() should ensure that vma_start_read() is using
the original mm and after locking the vma it should ensure that vma->vm_mm
has not changed from under us.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
---
 include/linux/mm.h | 12 ++++++++----
 mm/memory.c        |  7 ++++---
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 327cf5944569..88693568c9ef 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -739,10 +739,13 @@ static inline void vma_refcount_put(struct vm_area_struct *vma)
  * Try to read-lock a vma. The function is allowed to occasionally yield false
  * locked result to avoid performance overhead, in which case we fall back to
  * using mmap_lock. The function should never yield false unlocked result.
+ * False locked result is possible if mm_lock_seq overflows or if vma gets
+ * reused and attached to a different mm before we lock it.
  * Returns the vma on success, NULL on failure to lock and EAGAIN if vma got
  * detached.
  */
-static inline struct vm_area_struct *vma_start_read(struct vm_area_struct *vma)
+static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm,
+						    struct vm_area_struct *vma)
 {
 	int oldcnt;
 
@@ -753,7 +756,7 @@ static inline struct vm_area_struct *vma_start_read(struct vm_area_struct *vma)
 	 * we don't rely on for anything - the mm_lock_seq read against which we
 	 * need ordering is below.
 	 */
-	if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq.sequence))
+	if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(mm->mm_lock_seq.sequence))
 		return NULL;
 
 	/*
@@ -780,7 +783,7 @@ static inline struct vm_area_struct *vma_start_read(struct vm_area_struct *vma)
 	 * after it has been unlocked.
 	 * This pairs with RELEASE semantics in vma_end_write_all().
 	 */
-	if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) {
+	if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&mm->mm_lock_seq))) {
 		vma_refcount_put(vma);
 		return NULL;
 	}
@@ -914,7 +917,8 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 #else /* CONFIG_PER_VMA_LOCK */
 
 static inline void vma_lock_init(struct vm_area_struct *vma, bool reset_refcnt) {}
-static inline struct vm_area_struct *vma_start_read(struct vm_area_struct *vma)
+static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm,
+						    struct vm_area_struct *vma)
 		{ return NULL; }
 static inline void vma_end_read(struct vm_area_struct *vma) {}
 static inline void vma_start_write(struct vm_area_struct *vma) {}
diff --git a/mm/memory.c b/mm/memory.c
index 528407c0d7cf..6378a873e7c1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6492,7 +6492,7 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	if (!vma)
 		goto inval;
 
-	vma = vma_start_read(vma);
+	vma = vma_start_read(mm, vma);
 	if (IS_ERR_OR_NULL(vma)) {
 		/* Check if the VMA got isolated after we found it */
 		if (PTR_ERR(vma) == -EAGAIN) {
@@ -6512,8 +6512,9 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	 * fields are accessible for RCU readers.
 	 */
 
-	/* Check since vm_start/vm_end might change before we lock the VMA */
-	if (unlikely(address < vma->vm_start || address >= vma->vm_end))
+	/* Check if the vma we locked is the right one. */
+	if (unlikely(vma->vm_mm != mm ||
+		     address < vma->vm_start || address >= vma->vm_end))
 		goto inval_end_read;
 
 	rcu_read_unlock();
-- 
2.48.1.601.g30ceb7b040-goog