[PATCH v3 1/2] mm: limit the scope of vma_start_read()

[PATCH v3 1/2] mm: limit the scope of vma_start_read()

[Suren Baghdasaryan]

Limit the scope of vma_start_read() as it is used only as a helper for
higher-level locking functions implemented inside mmap_lock.c and we are
about to introduce more complex RCU rules for this function.
The change is pure code refactoring and has no functional changes.

Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
---
 include/linux/mmap_lock.h | 85 ---------------------------------------
 mm/mmap_lock.c            | 85 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 85 insertions(+), 85 deletions(-)

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 11a078de9150..2c9fffa58714 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -147,91 +147,6 @@ 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.
- *
- * WARNING! The vma passed to this function cannot be used if the function
- * fails to lock it because in certain cases RCU lock is dropped and then
- * reacquired. Once RCU lock is dropped the vma can be concurently freed.
- */
-static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm,
-						    struct vm_area_struct *vma)
-{
-	int oldcnt;
-
-	/*
-	 * 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))
-		return NULL;
-
-	/*
-	 * 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 */
-		return oldcnt ? NULL : ERR_PTR(-EAGAIN);
-	}
-
-	rwsem_acquire_read(&vma->vmlock_dep_map, 0, 1, _RET_IP_);
-
-	/*
-	 * 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.
-	 */
-	if (unlikely(vma->vm_mm != mm)) {
-		/* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt */
-		struct mm_struct *other_mm = vma->vm_mm;
-
-		/*
-		 * __mmdrop() is a heavy operation and we don't need RCU
-		 * protection here. Release RCU lock during these operations.
-		 * We reinstate the RCU read lock as the caller expects it to
-		 * be held when this function returns even on error.
-		 */
-		rcu_read_unlock();
-		mmgrab(other_mm);
-		vma_refcount_put(vma);
-		mmdrop(other_mm);
-		rcu_read_lock();
-		return NULL;
-	}
-
-	/*
-	 * 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);
-		return NULL;
-	}
-
-	return vma;
-}
-
 /*
  * 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
diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
index b006cec8e6fe..10826f347a9f 100644
--- a/mm/mmap_lock.c
+++ b/mm/mmap_lock.c
@@ -127,6 +127,91 @@ 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.
+ *
+ * WARNING! The vma passed to this function cannot be used if the function
+ * fails to lock it because in certain cases RCU lock is dropped and then
+ * reacquired. Once RCU lock is dropped the vma can be concurently freed.
+ */
+static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm,
+						    struct vm_area_struct *vma)
+{
+	int oldcnt;
+
+	/*
+	 * 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))
+		return NULL;
+
+	/*
+	 * 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 */
+		return oldcnt ? NULL : ERR_PTR(-EAGAIN);
+	}
+
+	rwsem_acquire_read(&vma->vmlock_dep_map, 0, 1, _RET_IP_);
+
+	/*
+	 * 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.
+	 */
+	if (unlikely(vma->vm_mm != mm)) {
+		/* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt */
+		struct mm_struct *other_mm = vma->vm_mm;
+
+		/*
+		 * __mmdrop() is a heavy operation and we don't need RCU
+		 * protection here. Release RCU lock during these operations.
+		 * We reinstate the RCU read lock as the caller expects it to
+		 * be held when this function returns even on error.
+		 */
+		rcu_read_unlock();
+		mmgrab(other_mm);
+		vma_refcount_put(vma);
+		mmdrop(other_mm);
+		rcu_read_lock();
+		return NULL;
+	}
+
+	/*
+	 * 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);
+		return NULL;
+	}
+
+	return vma;
+}
+
 /*
  * 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

base-commit: 01da54f10fddf3b01c5a3b80f6b16bbad390c302
-- 
2.50.1.565.gc32cd1483b-goog

[PATCH v3 2/2] mm: change vma_start_read() to drop RCU lock on failure

[Suren Baghdasaryan]

vma_start_read() can drop and reacquire RCU lock in certain failure
cases. It's not apparent that the RCU session started by the caller of
this function might be interrupted when vma_start_read() fails to lock
the vma. This might become a source of subtle bugs and to prevent that
we change the locking rules for vma_start_read() to drop RCU read lock
upon failure. This way it's more obvious that RCU-protected objects are
unsafe after vma locking fails.

Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Tested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
Changes since v2 [1]:
- Addressed nit about comment placement, per Vlastimil Babka
- Added Reviewed-by and Tested-by, per Lorenzo Stoakes and
Vlastimil Babka

[1] https://lore.kernel.org/all/20250731151919.212829-2-surenb@google.com/

 mm/mmap_lock.c | 84 +++++++++++++++++++++++++++-----------------------
 1 file changed, 45 insertions(+), 39 deletions(-)

diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
index 10826f347a9f..0a0db5849b8e 100644
--- a/mm/mmap_lock.c
+++ b/mm/mmap_lock.c
@@ -136,15 +136,16 @@ void vma_mark_detached(struct vm_area_struct *vma)
  * Returns the vma on success, NULL on failure to lock and EAGAIN if vma got
  * detached.
  *
- * WARNING! The vma passed to this function cannot be used if the function
- * fails to lock it because in certain cases RCU lock is dropped and then
- * reacquired. Once RCU lock is dropped the vma can be concurently freed.
+ * 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
@@ -152,8 +153,10 @@ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm,
 	 * 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))
-		return NULL;
+	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()
@@ -164,34 +167,14 @@ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm,
 	if (unlikely(!__refcount_inc_not_zero_limited_acquire(&vma->vm_refcnt, &oldcnt,
 							      VMA_REF_LIMIT))) {
 		/* return EAGAIN if vma got detached from under us */
-		return oldcnt ? NULL : ERR_PTR(-EAGAIN);
+		vma = oldcnt ? NULL : ERR_PTR(-EAGAIN);
+		goto err;
 	}
 
 	rwsem_acquire_read(&vma->vmlock_dep_map, 0, 1, _RET_IP_);
 
-	/*
-	 * 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.
-	 */
-	if (unlikely(vma->vm_mm != mm)) {
-		/* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt */
-		struct mm_struct *other_mm = vma->vm_mm;
-
-		/*
-		 * __mmdrop() is a heavy operation and we don't need RCU
-		 * protection here. Release RCU lock during these operations.
-		 * We reinstate the RCU read lock as the caller expects it to
-		 * be held when this function returns even on error.
-		 */
-		rcu_read_unlock();
-		mmgrab(other_mm);
-		vma_refcount_put(vma);
-		mmdrop(other_mm);
-		rcu_read_lock();
-		return NULL;
-	}
+	if (unlikely(vma->vm_mm != mm))
+		goto err_unstable;
 
 	/*
 	 * Overflow of vm_lock_seq/mm_lock_seq might produce false locked result.
@@ -206,10 +189,31 @@ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm,
 	 */
 	if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&mm->mm_lock_seq))) {
 		vma_refcount_put(vma);
-		return NULL;
+		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;
 }
 
 /*
@@ -223,11 +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)) {
@@ -247,18 +253,17 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	 * 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;
 }
@@ -313,6 +318,7 @@ struct vm_area_struct *lock_next_vma(struct mm_struct *mm,
 		 */
 		if (PTR_ERR(vma) == -EAGAIN) {
 			/* reset to search from the last address */
+			rcu_read_lock();
 			vma_iter_set(vmi, from_addr);
 			goto retry;
 		}
@@ -342,9 +348,9 @@ struct vm_area_struct *lock_next_vma(struct mm_struct *mm,
 	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 */
-- 
2.50.1.565.gc32cd1483b-goog