[PATCH v3 2/8] mm/vma: document possible vma->vm_refcnt values and reference comment

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

The possible vma->vm_refcnt values are confusing and vague, explain in
detail what these can be in a comment describing the vma->vm_refcnt field
and reference this comment in various places that read/write this field.

No functional change intended.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 include/linux/mm_types.h  | 39 +++++++++++++++++++++++++++++++++++++--
 include/linux/mmap_lock.h |  7 +++++++
 mm/mmap_lock.c            |  6 ++++++
 3 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 94de392ed3c5..e5ee66f84d9a 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -758,7 +758,8 @@ static inline struct anon_vma_name *anon_vma_name_alloc(const char *name)
  * set the VM_REFCNT_EXCLUDE_READERS_FLAG in vma->vm_refcnt to indiciate to
  * vma_start_read() that the reference count should be left alone.
  *
- * Once the operation is complete, this value is subtracted from vma->vm_refcnt.
+ * See the comment describing vm_refcnt in vm_area_struct for details as to
+ * which values the VMA reference count can be.
  */
 #define VM_REFCNT_EXCLUDE_READERS_BIT	(30)
 #define VM_REFCNT_EXCLUDE_READERS_FLAG	(1U << VM_REFCNT_EXCLUDE_READERS_BIT)
@@ -989,7 +990,41 @@ struct vm_area_struct {
 	struct vma_numab_state *numab_state;	/* NUMA Balancing state */
 #endif
 #ifdef CONFIG_PER_VMA_LOCK
-	/* Unstable RCU readers are allowed to read this. */
+	/*
+	 * Used to keep track of the number of references taken by VMA read or
+	 * write locks. May have the VM_REFCNT_EXCLUDE_READERS_FLAG set
+	 * indicating that a thread has entered __vma_enter_locked() and is
+	 * waiting on any outstanding read locks to exit.
+	 *
+	 * This value can be equal to:
+	 *
+	 * 0 - Detached.
+	 *
+	 * 1 - Unlocked or write-locked.
+	 *
+	 * >1, < VM_REFCNT_EXCLUDE_READERS_FLAG - Read-locked or (unlikely)
+	 * write-locked with other threads having temporarily incremented the
+	 * reference count prior to determining it is write-locked and
+	 * decrementing it again.
+	 *
+	 * VM_REFCNT_EXCLUDE_READERS_FLAG - Detached, pending
+	 * __vma_exit_locked() completion which will decrement the reference
+	 * count to zero. IMPORTANT - at this stage no further readers can
+	 * increment the reference count. It can only be reduced.
+	 *
+	 * VM_REFCNT_EXCLUDE_READERS_FLAG + 1 - Either an attached VMA pending
+	 * __vma_exit_locked() completion which will decrement the reference
+	 * count to one, OR a detached VMA waiting on a single spurious reader
+	 * to decrement reference count. IMPORTANT - as above, no further
+	 * readers can increment the reference count.
+	 *
+	 * > VM_REFCNT_EXCLUDE_READERS_FLAG + 1 - VMA is waiting on readers,
+	 * whether it is attempting to acquire a write lock or attempting to
+	 * detach. IMPORTANT - as above, no ruther readers can increment the
+	 * reference count.
+	 *
+	 * NOTE: Unstable RCU readers are allowed to read this.
+	 */
 	refcount_t vm_refcnt ____cacheline_aligned_in_smp;
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map vmlock_dep_map;
diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 5acbd4ba1b52..a764439d0276 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -130,6 +130,9 @@ static inline bool is_vma_writer_only(int refcnt)
 	 * attached. Waiting on a detached vma happens only in
 	 * vma_mark_detached() and is a rare case, therefore most of the time
 	 * there will be no unnecessary wakeup.
+	 *
+	 * See the comment describing the vm_area_struct->vm_refcnt field for
+	 * details of possible refcnt values.
 	 */
 	return (refcnt & VM_REFCNT_EXCLUDE_READERS_FLAG) &&
 		refcnt <= VM_REFCNT_EXCLUDE_READERS_FLAG + 1;
@@ -249,6 +252,10 @@ static inline void vma_assert_locked(struct vm_area_struct *vma)
 {
 	unsigned int mm_lock_seq;
 
+	/*
+	 * See the comment describing the vm_area_struct->vm_refcnt field for
+	 * details of possible refcnt values.
+	 */
 	VM_BUG_ON_VMA(refcount_read(&vma->vm_refcnt) <= 1 &&
 		      !__is_vma_write_locked(vma, &mm_lock_seq), vma);
 }
diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
index 1d23b48552e9..75dc098aea14 100644
--- a/mm/mmap_lock.c
+++ b/mm/mmap_lock.c
@@ -65,6 +65,9 @@ static inline int __vma_enter_locked(struct vm_area_struct *vma,
 	/*
 	 * If vma is detached then only vma_mark_attached() can raise the
 	 * vm_refcnt. mmap_write_lock prevents racing with vma_mark_attached().
+	 *
+	 * See the comment describing the vm_area_struct->vm_refcnt field for
+	 * details of possible refcnt values.
 	 */
 	if (!refcount_add_not_zero(VM_REFCNT_EXCLUDE_READERS_FLAG, &vma->vm_refcnt))
 		return 0;
@@ -137,6 +140,9 @@ void vma_mark_detached(struct vm_area_struct *vma)
 	 * before they check vm_lock_seq, realize the vma is locked and drop
 	 * back the vm_refcnt. That is a narrow window for observing a raised
 	 * vm_refcnt.
+	 *
+	 * See the comment describing the vm_area_struct->vm_refcnt field for
+	 * details of possible refcnt values.
 	 */
 	if (unlikely(!refcount_dec_and_test(&vma->vm_refcnt))) {
 		/* Wait until vma is detached with no readers. */
-- 
2.52.0