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Index: head/sys/vm/vm_page.h
===================================================================
--- head/sys/vm/vm_page.h
+++ head/sys/vm/vm_page.h
@@ -171,34 +171,32 @@
* The page daemon must therefore handle the possibility of a concurrent
* free of the page.
*
- * The queue field is the index of the page queue containing the page,
- * or PQ_NONE if the page is not enqueued. The queue lock of a page is
- * the page queue lock corresponding to the page queue index, or the
- * page lock (P) for the page if it is not enqueued. To modify the
- * queue field, the queue lock for the old value of the field must be
- * held. There is one exception to this rule: the page daemon may
- * transition the queue field from PQ_INACTIVE to PQ_NONE immediately
- * prior to freeing a page during an inactive queue scan. At that
- * point the page has already been physically dequeued and no other
- * references to that vm_page structure exist.
+ * The queue state of a page consists of the queue and act_count fields of
+ * its atomically updated state, and the subset of atomic flags specified
+ * by PGA_QUEUE_STATE_MASK. The queue field contains the page's page queue
+ * index, or PQ_NONE if it does not belong to a page queue. To modify the
+ * queue field, the page queue lock corresponding to the old value must be
+ * held, unless that value is PQ_NONE, in which case the queue index must
+ * be updated using an atomic RMW operation. There is one exception to
+ * this rule: the page daemon may transition the queue field from
+ * PQ_INACTIVE to PQ_NONE immediately prior to freeing the page during an
+ * inactive queue scan. At that point the page is already dequeued and no
+ * other references to that vm_page structure can exist. The PGA_ENQUEUED
+ * flag, when set, indicates that the page structure is physically inserted
+ * into the queue corresponding to the page's queue index, and may only be
+ * set or cleared with the corresponding page queue lock held.
*
- * To avoid contention on page queue locks, page queue operations
- * (enqueue, dequeue, requeue) are batched using per-CPU queues. A
- * deferred operation is requested by inserting an entry into a batch
- * queue; the entry is simply a pointer to the page, and the request
- * type is encoded in the page's aflags field using the values in
- * PGA_QUEUE_STATE_MASK. The type-stability of struct vm_pages is
- * crucial to this scheme since the processing of entries in a given
- * batch queue may be deferred indefinitely. In particular, a page may
- * be freed before its pending batch queue entries have been processed.
- * The page lock (P) must be held to schedule a batched queue
- * operation, and the page queue lock must be held in order to process
- * batch queue entries for the page queue. There is one exception to
- * this rule: the thread freeing a page may schedule a dequeue without
- * holding the page lock. In this scenario the only other thread which
- * may hold a reference to the page is the page daemon, which is
- * careful to avoid modifying the page's queue state once the dequeue
- * has been requested by setting PGA_DEQUEUE.
+ * To avoid contention on page queue locks, page queue operations (enqueue,
+ * dequeue, requeue) are batched using fixed-size per-CPU queues. A
+ * deferred operation is requested by setting one of the flags in
+ * PGA_QUEUE_OP_MASK and inserting an entry into a batch queue. When a
+ * queue is full, an attempt to insert a new entry will lock the page
+ * queues and trigger processing of the pending entries. The
+ * type-stability of vm_page structures is crucial to this scheme since the
+ * processing of entries in a given batch queue may be deferred
+ * indefinitely. In particular, a page may be freed with pending batch
+ * queue entries. The page queue operation flags must be set using atomic
+ * RWM operations.
*/
#if PAGE_SIZE == 4096

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