1 files changed, 100 insertions, 9 deletions

diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
index b006cec8e6fe..0a0db5849b8e 100644
--- a/mm/mmap_lock.c
+++ b/mm/mmap_lock.c
@@ -128,6 +128,95 @@ void vma_mark_detached(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.
+ *
+ * IMPORTANT: RCU lock must be held upon entering the function, but upon error
+ *            IT IS RELEASED. The caller must handle this correctly.
+ */
+static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm,
+						    struct vm_area_struct *vma)
+{
+	struct mm_struct *other_mm;
+	int oldcnt;
+
+	RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "no rcu lock held");
+	/*
+	 * Check before locking. A race might cause false locked result.
+	 * We can use READ_ONCE() for the mm_lock_seq here, and don't need
+	 * ACQUIRE semantics, because this is just a lockless check whose result
+	 * 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(mm->mm_lock_seq.sequence)) {
+		vma = NULL;
+		goto err;
+	}
+
+	/*
+	 * If VMA_LOCK_OFFSET is set, __refcount_inc_not_zero_limited_acquire()
+	 * will fail because VMA_REF_LIMIT is less than VMA_LOCK_OFFSET.
+	 * Acquire fence is required here to avoid reordering against later
+	 * vm_lock_seq check and checks inside lock_vma_under_rcu().
+	 */
+	if (unlikely(!__refcount_inc_not_zero_limited_acquire(&vma->vm_refcnt, &oldcnt,
+							      VMA_REF_LIMIT))) {
+		/* return EAGAIN if vma got detached from under us */
+		vma = oldcnt ? NULL : ERR_PTR(-EAGAIN);
+		goto err;
+	}
+
+	rwsem_acquire_read(&vma->vmlock_dep_map, 0, 1, _RET_IP_);
+
+	if (unlikely(vma->vm_mm != mm))
+		goto err_unstable;
+
+	/*
+	 * Overflow of vm_lock_seq/mm_lock_seq might produce false locked result.
+	 * False unlocked result is impossible because we modify and check
+	 * vma->vm_lock_seq under vma->vm_refcnt protection and mm->mm_lock_seq
+	 * modification invalidates all existing locks.
+	 *
+	 * We must use ACQUIRE semantics for the mm_lock_seq so that if we are
+	 * racing with vma_end_write_all(), we only start reading from the 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(&mm->mm_lock_seq))) {
+		vma_refcount_put(vma);
+		vma = NULL;
+		goto err;
+	}
+
+	return vma;
+err:
+	rcu_read_unlock();
+
+	return vma;
+err_unstable:
+	/*
+	 * If vma got attached to another mm from under us, that mm is not
+	 * stable and can be freed in the narrow window after vma->vm_refcnt
+	 * is dropped and before rcuwait_wake_up(mm) is called. Grab it before
+	 * releasing vma->vm_refcnt.
+	 */
+	other_mm = vma->vm_mm; /* use a copy as vma can be freed after we drop vm_refcnt */
+
+	/* __mmdrop() is a heavy operation, do it after dropping RCU lock. */
+	rcu_read_unlock();
+	mmgrab(other_mm);
+	vma_refcount_put(vma);
+	mmdrop(other_mm);
+
+	return NULL;
+}
+
+/*
  * Lookup and lock a VMA under RCU protection. Returned VMA is guaranteed to be
  * stable and not isolated. If the VMA is not found or is being modified the
  * function returns NULL.
@@ -138,11 +227,13 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	MA_STATE(mas, &mm->mm_mt, address, address);
 	struct vm_area_struct *vma;
 
-	rcu_read_lock();
 retry:
+	rcu_read_lock();
 	vma = mas_walk(&mas);
-	if (!vma)
+	if (!vma) {
+		rcu_read_unlock();
 		goto inval;
+	}
 
 	vma = vma_start_read(mm, vma);
 	if (IS_ERR_OR_NULL(vma)) {
@@ -162,18 +253,17 @@ retry:
 	 * From here on, we can access the VMA without worrying about which
 	 * fields are accessible for RCU readers.
 	 */
+	rcu_read_unlock();
 
 	/* Check if the vma we locked is the right one. */
-	if (unlikely(address < vma->vm_start || address >= vma->vm_end))
-		goto inval_end_read;
+	if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
+		vma_end_read(vma);
+		goto inval;
+	}
 
-	rcu_read_unlock();
 	return vma;
 
-inval_end_read:
-	vma_end_read(vma);
 inval:
-	rcu_read_unlock();
 	count_vm_vma_lock_event(VMA_LOCK_ABORT);
 	return NULL;
 }
@@ -228,6 +318,7 @@ retry:
 		 */
 		if (PTR_ERR(vma) == -EAGAIN) {
 			/* reset to search from the last address */
+			rcu_read_lock();
 			vma_iter_set(vmi, from_addr);
 			goto retry;
 		}
@@ -257,9 +348,9 @@ retry:
 	return vma;
 
 fallback_unlock:
+	rcu_read_unlock();
 	vma_end_read(vma);
 fallback:
-	rcu_read_unlock();
 	vma = lock_next_vma_under_mmap_lock(mm, vmi, from_addr);
 	rcu_read_lock();
 	/* Reinitialize the iterator after re-entering rcu read section */