Re: [PATCH 1/2] maple_tree: Fix mas_skip_node() end slot detection

From: Peng Zhang
Date: Tue Mar 07 2023 - 11:17:30 EST


Hi, Snild,

在 2023/3/7 22:30, Snild Dolkow 写道:
On 2023-03-07 14:05, Peng Zhang wrote:
Hi, Liam,
-    } while (slot > slot_count);
+    } while (mas->offset >= mas_data_end(mas));
-    mas->offset = ++slot;
+    mt = mte_node_type(mas->node);
      pivots = ma_pivots(mas_mn(mas), mt);
-    if (slot > 0)
-        mas->min = pivots[slot - 1] + 1;
-
-    if (slot <= slot_count)
-        mas->max = pivots[slot];
+    mas->min = pivots[mas->offset] + 1;
+    mas->offset++;
+    if (mas->offset < mt_slots[mt])
+        mas->max = pivots[mas->offset];
There is a bug here, the assignment of mas->min and mas->max is wrong.
The assignment will make them represent the range of a child node, but it should represent the range of the current node. After mas_ascend() returns, mas-min and mas->max already represent the range of the current node, so we should delete these assignments of mas->min and mas->max.


Thanks for your suggestion, Peng. Applying it literally by removing only the min/max assignments:

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 6fc1ad42b409..9b6e581cf83f 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -5118,10 +5118,7 @@ static inline bool mas_skip_node

        mt = mte_node_type(mas->node);
        pivots = ma_pivots(mas_mn(mas), mt);
-       mas->min = pivots[mas->offset] + 1;
        mas->offset++;
-       if (mas->offset < mt_slots[mt])
-               mas->max = pivots[mas->offset];

        return true;
 }


This allowed my test to pass 100/100 runs. Still in qemu with the test as init, so not really stressed in any way except that specific usecase.

//Snild

Thanks for the test, I'm happy if it happens to fix your problem. So a patch was made.
This patch needs to be applied after Liam's patch.

Sincerely yours,
Peng.