Subject: [PATCH] maple_tree: Fix mas_prev() state separation code

From: "Liam R. Howlett" <[email-masked]>
Date: Thu, 7 Dec 2023 14:33:19 -0500

mas_prev() was setting the ma_underflow condition when the limit was
reached and not when the limit was attempting to go lower.  This
resulted in the incorrect behaviour on subsequent actions.

This commit fixes the status setting to only set ma_underflow when the
lower limit is attempted to be decremented, and modifies the testing to
ensure that's the case.

Signed-off-by: Liam R. Howlett <[email-masked]>
---

Andrew,

This should be clean to squash into 7f79fdb1d94d7 ("maple_tree: separate
ma_state node from status.") which is currently in your mm-unstable
branch.

Thanks,
Liam

 lib/maple_tree.c      | 16 ++++++++++++----
 lib/test_maple_tree.c |  9 +++++++--
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 89f8d21602774..47f2a7a973852 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -4432,6 +4432,9 @@ static void *mas_prev_slot(struct ma_state *mas, unsigned long min, bool empty)
 		mas->last = mas->index - 1;
 		mas->index = mas_safe_min(mas, pivots, mas->offset);
 	} else  {
+		if (mas->index <= min)
+			goto underflow;
+
 		if (mas_prev_node(mas, min)) {
 			mas_rewalk(mas, save_point);
 			goto retry;
@@ -4452,15 +4455,15 @@ static void *mas_prev_slot(struct ma_state *mas, unsigned long min, bool empty)
 	if (unlikely(mas_rewalk_if_dead(mas, node, save_point)))
 		goto retry;
 
-	if (mas->index <= min)
-		mas->status = ma_underflow;
 
 	if (likely(entry))
 		return entry;
 
 	if (!empty) {
-		if (mas_is_underflow(mas))
+		if (mas->index <= min) {
+			mas->status = ma_underflow;
 			return NULL;
+		}
 
 		goto again;
 	}
@@ -4596,7 +4599,7 @@ static void *mas_next_slot(struct ma_state *mas, unsigned long max, bool empty)
 		if (unlikely(mas_rewalk_if_dead(mas, node, save_point)))
 			goto retry;
 
-		if (pivot >= max) {
+		if (pivot >= max) { /* Was at the limit, next will extend beyond */
 			mas->status = ma_overflow;
 			return NULL;
 		}
@@ -4611,6 +4614,11 @@ static void *mas_next_slot(struct ma_state *mas, unsigned long max, bool empty)
 		else
 			mas->last = mas->max;
 	} else  {
+		if (mas->last >= max) {
+			mas->status = ma_overflow;
+			return NULL;
+		}
+
 		if (mas_next_node(mas, node, max)) {
 			mas_rewalk(mas, save_point);
 			goto retry;
diff --git a/lib/test_maple_tree.c b/lib/test_maple_tree.c
index 15fbeb788f3ac..29185ac5c727f 100644
--- a/lib/test_maple_tree.c
+++ b/lib/test_maple_tree.c
@@ -3294,8 +3294,8 @@ static noinline void __init check_state_handling(struct maple_tree *mt)
 	MT_BUG_ON(mt, mas.last != 0x1500);
 	MT_BUG_ON(mt, !mas_is_active(&mas));
 
-	/* next:active ->active */
-	entry = mas_next(&mas, ULONG_MAX);
+	/* next:active ->active (spanning limit) */
+	entry = mas_next(&mas, 0x2100);
 	MT_BUG_ON(mt, entry != ptr2);
 	MT_BUG_ON(mt, mas.index != 0x2000);
 	MT_BUG_ON(mt, mas.last != 0x2500);
@@ -3360,6 +3360,11 @@ static noinline void __init check_state_handling(struct maple_tree *mt)
 	MT_BUG_ON(mt, entry != ptr);
 	MT_BUG_ON(mt, mas.index != 0x1000);
 	MT_BUG_ON(mt, mas.last != 0x1500);
+	MT_BUG_ON(mt, !mas_is_active(&mas)); /* spanning limit */
+	entry = mas_prev(&mas, 0x1200); /* underflow */
+	MT_BUG_ON(mt, entry != NULL);
+	MT_BUG_ON(mt, mas.index != 0x1000);
+	MT_BUG_ON(mt, mas.last != 0x1500);
 	MT_BUG_ON(mt, !mas_is_underflow(&mas));
 
 	/* prev:underflow -> underflow (lower limit) spanning end range */
-- 
2.40.1