[PATCH v6 14/16] 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>
---
 include/linux/mm.h | 10 ++++++----
 mm/memory.c        |  7 ++++---
 2 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 361f26dedab1..bfd01ae07660 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -725,8 +725,10 @@ 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.
  */
-static inline bool vma_start_read(struct vm_area_struct *vma)
+static inline bool vma_start_read(struct mm_struct *mm, struct vm_area_struct *vma)
 {
 	int oldcnt;
 
@@ -737,7 +739,7 @@ static inline bool 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 false;
 
 
@@ -762,7 +764,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
 	 * This pairs with RELEASE semantics in vma_end_write_all().
 	 */
 	if (oldcnt & VMA_STATE_LOCKED ||
-	    unlikely(vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) {
+	    unlikely(vma->vm_lock_seq == raw_read_seqcount(&mm->mm_lock_seq))) {
 		vma_refcount_put(vma);
 		return false;
 	}
@@ -918,7 +920,7 @@ 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) {}
-static inline bool vma_start_read(struct vm_area_struct *vma)
+static inline bool vma_start_read(struct mm_struct *mm, struct vm_area_struct *vma)
 		{ return false; }
 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 534e279f98c1..2131d9769bb4 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6405,7 +6405,7 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	if (!vma)
 		goto inval;
 
-	if (!vma_start_read(vma))
+	if (!vma_start_read(mm, vma))
 		goto inval;
 
 	/*
@@ -6415,8 +6415,9 @@ 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();
-- 
2.47.1.613.gc27f4b7a9f-goog