dm bufio: add LRU abstraction

A CLOCK algorithm is used in this LRU abstraction.  This avoids
relinking list nodes, which would require a write lock protecting it.

None of the LRU methods are threadsafe; locking must be done at a
higher level.

Code that uses this new LRU will be introduced in the next 2 commits.

As such, this commit will cause "defined but not used" compiler warnings
that will be resolved by the next 2 commits.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-bufio.c

@@ -66,6 +66,241 @@
 #define LIST_DIRTY	1
 #define LIST_SIZE	2
 
+/*--------------------------------------------------------------*/
+
+/*
+ * Rather than use an LRU list, we use a clock algorithm where entries
+ * are held in a circular list.  When an entry is 'hit' a reference bit
+ * is set.  The least recently used entry is approximated by running a
+ * cursor around the list selecting unreferenced entries. Referenced
+ * entries have their reference bit cleared as the cursor passes them.
+ */
+struct lru_entry {
+	struct list_head list;
+	atomic_t referenced;
+};
+
+struct lru_iter {
+	struct lru *lru;
+	struct list_head list;
+	struct lru_entry *stop;
+	struct lru_entry *e;
+};
+
+struct lru {
+	struct list_head *cursor;
+	unsigned long count;
+
+	struct list_head iterators;
+};
+
+/*--------------*/
+
+static void lru_init(struct lru *lru)
+{
+	lru->cursor = NULL;
+	lru->count = 0;
+	INIT_LIST_HEAD(&lru->iterators);
+}
+
+static void lru_destroy(struct lru *lru)
+{
+	WARN_ON_ONCE(lru->cursor);
+	WARN_ON_ONCE(!list_empty(&lru->iterators));
+}
+
+/*
+ * Insert a new entry into the lru.
+ */
+static void lru_insert(struct lru *lru, struct lru_entry *le)
+{
+	/*
+	 * Don't be tempted to set to 1, makes the lru aspect
+	 * perform poorly.
+	 */
+	atomic_set(&le->referenced, 0);
+
+	if (lru->cursor) {
+		list_add_tail(&le->list, lru->cursor);
+	} else {
+		INIT_LIST_HEAD(&le->list);
+		lru->cursor = &le->list;
+	}
+	lru->count++;
+}
+
+/*--------------*/
+
+/*
+ * Convert a list_head pointer to an lru_entry pointer.
+ */
+static inline struct lru_entry *to_le(struct list_head *l)
+{
+	return container_of(l, struct lru_entry, list);
+}
+
+/*
+ * Initialize an lru_iter and add it to the list of cursors in the lru.
+ */
+static void lru_iter_begin(struct lru *lru, struct lru_iter *it)
+{
+	it->lru = lru;
+	it->stop = lru->cursor ? to_le(lru->cursor->prev) : NULL;
+	it->e = lru->cursor ? to_le(lru->cursor) : NULL;
+	list_add(&it->list, &lru->iterators);
+}
+
+/*
+ * Remove an lru_iter from the list of cursors in the lru.
+ */
+static inline void lru_iter_end(struct lru_iter *it)
+{
+	list_del(&it->list);
+}
+
+/* Predicate function type to be used with lru_iter_next */
+typedef bool (*iter_predicate)(struct lru_entry *le, void *context);
+
+/*
+ * Advance the cursor to the next entry that passes the
+ * predicate, and return that entry.  Returns NULL if the
+ * iteration is complete.
+ */
+static struct lru_entry *lru_iter_next(struct lru_iter *it,
+				       iter_predicate pred, void *context)
+{
+	struct lru_entry *e;
+
+	while (it->e) {
+		e = it->e;
+
+		/* advance the cursor */
+		if (it->e == it->stop)
+			it->e = NULL;
+		else
+			it->e = to_le(it->e->list.next);
+
+		if (pred(e, context))
+			return e;
+	}
+
+	return NULL;
+}
+
+/*
+ * Invalidate a specific lru_entry and update all cursors in
+ * the lru accordingly.
+ */
+static void lru_iter_invalidate(struct lru *lru, struct lru_entry *e)
+{
+	struct lru_iter *it;
+
+	list_for_each_entry(it, &lru->iterators, list) {
+		/* Move c->e forwards if necc. */
+		if (it->e == e) {
+			it->e = to_le(it->e->list.next);
+			if (it->e == e)
+				it->e = NULL;
+		}
+
+		/* Move it->stop backwards if necc. */
+		if (it->stop == e) {
+			it->stop = to_le(it->stop->list.prev);
+			if (it->stop == e)
+				it->stop = NULL;
+		}
+	}
+}
+
+/*--------------*/
+
+/*
+ * Remove a specific entry from the lru.
+ */
+static void lru_remove(struct lru *lru, struct lru_entry *le)
+{
+	lru_iter_invalidate(lru, le);
+	if (lru->count == 1) {
+		lru->cursor = NULL;
+	} else {
+		if (lru->cursor == &le->list)
+			lru->cursor = lru->cursor->next;
+		list_del(&le->list);
+	}
+	lru->count--;
+}
+
+/*
+ * Mark as referenced.
+ */
+static inline void lru_reference(struct lru_entry *le)
+{
+	atomic_set(&le->referenced, 1);
+}
+
+/*--------------*/
+
+/*
+ * Remove the least recently used entry (approx), that passes the predicate.
+ * Returns NULL on failure.
+ */
+enum evict_result {
+	ER_EVICT,
+	ER_DONT_EVICT,
+	ER_STOP, /* stop looking for something to evict */
+};
+
+typedef enum evict_result (*le_predicate)(struct lru_entry *le, void *context);
+
+static struct lru_entry *lru_evict(struct lru *lru, le_predicate pred, void *context)
+{
+	unsigned long tested = 0;
+	struct list_head *h = lru->cursor;
+	struct lru_entry *le;
+
+	if (!h)
+		return NULL;
+	/*
+	 * In the worst case we have to loop around twice. Once to clear
+	 * the reference flags, and then again to discover the predicate
+	 * fails for all entries.
+	 */
+	while (tested < lru->count) {
+		le = container_of(h, struct lru_entry, list);
+
+		if (atomic_read(&le->referenced)) {
+			atomic_set(&le->referenced, 0);
+		} else {
+			tested++;
+			switch (pred(le, context)) {
+			case ER_EVICT:
+				/*
+				 * Adjust the cursor, so we start the next
+				 * search from here.
+				 */
+				lru->cursor = le->list.next;
+				lru_remove(lru, le);
+				return le;
+
+			case ER_DONT_EVICT:
+				break;
+
+			case ER_STOP:
+				lru->cursor = le->list.next;
+				return NULL;
+			}
+		}
+
+		h = h->next;
+
+		cond_resched();
+	}
+
+	return NULL;
+}
+
+/*--------------------------------------------------------------*/
+
 /*
  * Linking of buffers:
  *	All buffers are linked to buffer_tree with their node field.