bpo-45521: Fix a bug in the obmalloc radix tree code. (GH-29051) (GH-29122)

MAP_BOT_LENGTH was incorrectly used to compute MAP_TOP_MASK instead of MAP_TOP_LENGTH. On 64-bit machines, the error causes the tree to hold 46-bits of virtual addresses, rather than the intended 48-bits. (cherry picked from commit 311910b)

Author: miss-islington

Misc/NEWS.d/next/Core and Builtins/2021-10-18-22-40-33.bpo-45521.GdMiuW.rst

@@ -0,0 +1,3 @@
+Fix a bug in the obmalloc radix tree code. On 64-bit machines, the bug
+causes the tree to hold 46-bits of virtual addresses, rather than the
+intended 48-bits.

Objects/obmalloc.c

@@ -1328,7 +1328,7 @@ _Py_GetAllocatedBlocks(void)
 
 #define MAP_TOP_BITS INTERIOR_BITS
 #define MAP_TOP_LENGTH (1 << MAP_TOP_BITS)
-#define MAP_TOP_MASK (MAP_BOT_LENGTH - 1)
+#define MAP_TOP_MASK (MAP_TOP_LENGTH - 1)
 
 #define MAP_MID_BITS INTERIOR_BITS
 #define MAP_MID_LENGTH (1 << MAP_MID_BITS)