maple_tree: introduce mas_prev_slot() interface

Sometimes the user needs to revert to the previous slot, regardless of if
it is empty or not.  Add an interface to go to the previous slot.

Since there can't be two consecutive NULLs in the tree, the mas_prev()
function can be implemented by calling mas_prev_slot() a maximum of 2
times.  Change the underlying interface to use mas_prev_slot() to align
the code.

Link: https://lkml.kernel.org/r/20230518145544.1722059-31-Liam.Howlett@oracle.comSigned-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: David Binderman <dcb314@hotmail.com>
Cc: Peng Zhang <zhangpeng.00@bytedance.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Vernon Yang <vernon2gm@gmail.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/maple_tree.c

@@ -4532,15 +4532,19 @@ static inline int mas_prev_node(struct ma_state *mas, unsigned long min)
 	int offset, level;
 	void __rcu **slots;
 	struct maple_node *node;
-	struct maple_enode *enode;
 	unsigned long *pivots;
+	unsigned long max;
 
-	if (mas_is_none(mas))
-		return 0;
+	node = mas_mn(mas);
+	if (!mas->min)
+		goto no_entry;
+
+	max = mas->min - 1;
+	if (max < min)
+		goto no_entry;
 
 	level = 0;
 	do {
-		node = mas_mn(mas);
 		if (ma_is_root(node))
 			goto no_entry;
 
@@ -4549,64 +4553,41 @@ static inline int mas_prev_node(struct ma_state *mas, unsigned long min)
 			return 1;
 		offset = mas->offset;
 		level++;
+		node = mas_mn(mas);
 	} while (!offset);
 
 	offset--;
 	mt = mte_node_type(mas->node);
-	node = mas_mn(mas);
-	slots = ma_slots(node, mt);
-	pivots = ma_pivots(node, mt);
-	if (unlikely(ma_dead_node(node)))
-		return 1;
-
-	mas->max = pivots[offset];
-	if (offset)
-		mas->min = pivots[offset - 1] + 1;
-	if (unlikely(ma_dead_node(node)))
-		return 1;
-
-	if (mas->max < min)
-		goto no_entry_min;
-
 	while (level > 1) {
 		level--;
-		enode = mas_slot(mas, slots, offset);
+		slots = ma_slots(node, mt);
+		mas->node = mas_slot(mas, slots, offset);
 		if (unlikely(ma_dead_node(node)))
 			return 1;
 
-		mas->node = enode;
 		mt = mte_node_type(mas->node);
 		node = mas_mn(mas);
-		slots = ma_slots(node, mt);
 		pivots = ma_pivots(node, mt);
-		offset = ma_data_end(node, mt, pivots, mas->max);
+		offset = ma_data_end(node, mt, pivots, max);
 		if (unlikely(ma_dead_node(node)))
 			return 1;
-
-		if (offset)
-			mas->min = pivots[offset - 1] + 1;
-
-		if (offset < mt_pivots[mt])
-			mas->max = pivots[offset];
-
-		if (mas->max < min)
-			goto no_entry;
 	}
 
+	slots = ma_slots(node, mt);
 	mas->node = mas_slot(mas, slots, offset);
+	pivots = ma_pivots(node, mt);
 	if (unlikely(ma_dead_node(node)))
 		return 1;
 
+	if (likely(offset))
+		mas->min = pivots[offset - 1] + 1;
+	mas->max = max;
 	mas->offset = mas_data_end(mas);
 	if (unlikely(mte_dead_node(mas->node)))
 		return 1;
 
 	return 0;
 
-no_entry_min:
-	mas->offset = offset;
-	if (offset)
-		mas->min = pivots[offset - 1] + 1;
 no_entry:
 	if (unlikely(ma_dead_node(node)))
 		return 1;
@@ -4615,6 +4596,76 @@ static inline int mas_prev_node(struct ma_state *mas, unsigned long min)
 	return 0;
 }
 
+/*
+ * mas_prev_slot() - Get the entry in the previous slot
+ *
+ * @mas: The maple state
+ * @max: The minimum starting range
+ *
+ * Return: The entry in the previous slot which is possibly NULL
+ */
+static void *mas_prev_slot(struct ma_state *mas, unsigned long min, bool empty)
+{
+	void *entry;
+	void __rcu **slots;
+	unsigned long pivot;
+	enum maple_type type;
+	unsigned long *pivots;
+	struct maple_node *node;
+	unsigned long save_point = mas->index;
+
+retry:
+	node = mas_mn(mas);
+	type = mte_node_type(mas->node);
+	pivots = ma_pivots(node, type);
+	if (unlikely(mas_rewalk_if_dead(mas, node, save_point)))
+		goto retry;
+
+again:
+	if (mas->min <= min) {
+		pivot = mas_safe_min(mas, pivots, mas->offset);
+
+		if (unlikely(mas_rewalk_if_dead(mas, node, save_point)))
+			goto retry;
+
+		if (pivot <= min)
+			return NULL;
+	}
+
+	if (likely(mas->offset)) {
+		mas->offset--;
+		mas->last = mas->index - 1;
+		mas->index = mas_safe_min(mas, pivots, mas->offset);
+	} else  {
+		if (mas_prev_node(mas, min)) {
+			mas_rewalk(mas, save_point);
+			goto retry;
+		}
+
+		if (mas_is_none(mas))
+			return NULL;
+
+		mas->last = mas->max;
+		node = mas_mn(mas);
+		type = mte_node_type(mas->node);
+		pivots = ma_pivots(node, type);
+		mas->index = pivots[mas->offset - 1] + 1;
+	}
+
+	slots = ma_slots(node, type);
+	entry = mas_slot(mas, slots, mas->offset);
+	if (unlikely(mas_rewalk_if_dead(mas, node, save_point)))
+		goto retry;
+
+	if (likely(entry))
+		return entry;
+
+	if (!empty)
+		goto again;
+
+	return entry;
+}
+
 /*
  * mas_next_node() - Get the next node at the same level in the tree.
  * @mas: The maple state
@@ -4799,109 +4850,6 @@ static inline void *mas_next_entry(struct ma_state *mas, unsigned long limit)
 	return mas_next_slot(mas, limit, false);
 }
 
-/*
- * mas_prev_nentry() - Get the previous node entry.
- * @mas: The maple state.
- * @limit: The lower limit to check for a value.
- *
- * Return: the entry, %NULL otherwise.
- */
-static inline void *mas_prev_nentry(struct ma_state *mas, unsigned long limit,
-				    unsigned long index)
-{
-	unsigned long pivot, min;
-	unsigned char offset, count;
-	struct maple_node *mn;
-	enum maple_type mt;
-	unsigned long *pivots;
-	void __rcu **slots;
-	void *entry;
-
-retry:
-	if (!mas->offset)
-		return NULL;
-
-	mn = mas_mn(mas);
-	mt = mte_node_type(mas->node);
-	offset = mas->offset - 1;
-	slots = ma_slots(mn, mt);
-	pivots = ma_pivots(mn, mt);
-	count = ma_data_end(mn, mt, pivots, mas->max);
-	if (unlikely(mas_rewalk_if_dead(mas, mn, index)))
-		goto retry;
-
-	offset = mas->offset - 1;
-	if (offset >= mt_slots[mt])
-		offset = mt_slots[mt] - 1;
-
-	if (offset >= count) {
-		pivot = mas->max;
-		offset = count;
-	} else {
-		pivot = pivots[offset];
-	}
-
-	if (unlikely(mas_rewalk_if_dead(mas, mn, index)))
-		goto retry;
-
-	while (offset && !mas_slot(mas, slots, offset)) {
-		pivot = pivots[--offset];
-		if (pivot >= limit)
-			break;
-	}
-
-	/*
-	 * If the slot was null but we've shifted outside the limits, then set
-	 * the range to the last NULL.
-	 */
-	if (unlikely((pivot < limit) && (offset < mas->offset)))
-		pivot = pivots[++offset];
-
-	min = mas_safe_min(mas, pivots, offset);
-	entry = mas_slot(mas, slots, offset);
-	if (unlikely(mas_rewalk_if_dead(mas, mn, index)))
-		goto retry;
-
-	mas->offset = offset;
-	mas->last = pivot;
-	mas->index = min;
-	return entry;
-}
-
-static inline void *mas_prev_entry(struct ma_state *mas, unsigned long min)
-{
-	void *entry;
-	struct maple_enode *prev_enode;
-	unsigned char prev_offset;
-
-	if (mas->index < min)
-		return NULL;
-
-retry:
-	prev_enode = mas->node;
-	prev_offset = mas->offset;
-	while (likely(!mas_is_none(mas))) {
-		entry = mas_prev_nentry(mas, min, mas->index);
-
-		if (likely(entry))
-			return entry;
-
-		if (unlikely(mas->index <= min))
-			return NULL;
-
-		if (unlikely(mas_prev_node(mas, min))) {
-			mas_rewalk(mas, mas->index);
-			goto retry;
-		}
-
-		mas->offset++;
-	}
-
-	mas->node = prev_enode;
-	mas->offset = prev_offset;
-	return NULL;
-}
-
 /*
  * mas_rev_awalk() - Internal function.  Reverse allocation walk.  Find the
  * highest gap address of a given size in a given node and descend.
@@ -6012,7 +5960,7 @@ void *mas_prev(struct ma_state *mas, unsigned long min)
 		}
 		return NULL;
 	}
-	return mas_prev_entry(mas, min);
+	return mas_prev_slot(mas, min, false);
 
 none:
 	mas->node = MAS_NONE;
@@ -6227,8 +6175,8 @@ void *mas_find_rev(struct ma_state *mas, unsigned long min)
 	if (mas->index < min)
 		return NULL;
 
-	/* Retries on dead nodes handled by mas_prev_entry */
-	return mas_prev_entry(mas, min);
+	/* Retries on dead nodes handled by mas_prev_slot */
+	return mas_prev_slot(mas, min, false);
 
 none:
 	mas->node = MAS_NONE;