mm/vma: document possible vma->vm_refcnt values and reference comment

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.

[akpm@linux-foundation.org: fix typo, per Suren]
Link: https://lkml.kernel.org/r/d462e7678c6cc7461f94e5b26c776547d80a67e8.1769198904.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Waiman Long <longman@redhat.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 47 insertions, 2 deletions

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index bdbf17c4f26b..3e608d22cab0 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 +990,44 @@ 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 firstly, whether the VMA is attached, secondly,
+	 * if attached, how many read locks are taken, and thirdly, if the
+	 * VM_REFCNT_EXCLUDE_READERS_FLAG is set, whether any read locks held
+	 * are currently in the process of being excluded.
+	 *
+	 * This value can be equal to:
+	 *
+	 * 0 - Detached. IMPORTANT: when the refcnt is zero, readers cannot
+	 * increment it.
+	 *
+	 * 1 - Attached and either unlocked or write-locked. Write locks are
+	 * identified via __is_vma_write_locked() which checks for equality of
+	 * vma->vm_lock_seq and mm->mm_lock_seq.
+	 *
+	 * >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 - A thread is either write-locking
+	 * an attached VMA and has yet to invoke __vma_exit_locked(), OR a
+	 * thread is detaching a VMA and is waiting on a single spurious reader
+	 * in order to decrement the reference count. IMPORTANT - as above, no
+	 * further readers can increment the reference count.
+	 *
+	 * > VM_REFCNT_EXCLUDE_READERS_FLAG + 1 - A thread is either
+	 * write-locking or detaching a VMA is waiting on readers to
+	 * exit. IMPORTANT - as above, no further 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);
 }