radix-tree: drop radix_tree_init_maxindex()

Radix trees are in principle usable as soon as memory allocation works.
(Radix trees with only index 0 populated are usable even earlier.) If only
there wasn't height_to_maxindex[], which is filled only by a pre-SMP
initcall. The benefit of this array is rather limited - the calculations
done by __maxindex() can as well be done by radix_tree_maxindex(); the
overhead isn't all this high.

This was found by UBSAN on a multi-segment system:

(XEN) UBSAN: Undefined behaviour in common/radix-tree.c:83:27
(XEN) index 12 is out of range for type 'long unsigned int [12]'
...
(XEN) Xen call trace:
(XEN)    [<ffff82d040323f9c>] R common/ubsan/ubsan.c#ubsan_epilogue+0xa/0xd5
(XEN)    [<ffff82d040324d91>] F __ubsan_handle_out_of_bounds+0x9d/0xd4
(XEN)    [<ffff82d04029265a>] F radix_tree_insert+0x24d/0x570
(XEN)    [<ffff82d04037ac3e>] F drivers/passthrough/pci.c#alloc_pseg+0xc4/0x165
(XEN)    [<ffff82d040a3b526>] F pci_add_segment+0xc/0x1b
(XEN)    [<ffff82d040a5ad1b>] F acpi_parse_mcfg+0x29b/0x344
(XEN)    [<ffff82d040a3f612>] F acpi_table_parse+0x5d/0x92
(XEN)    [<ffff82d040a5bf55>] F acpi_mmcfg_init+0x3a2/0x71d
(XEN)    [<ffff82d040a71ba6>] F pci_setup+0x17/0x29
(XEN)    [<ffff82d040a784d0>] F __start_xen+0x394c/0x4ed8
(XEN)    [<ffff82d040423057>] F __high_start+0xb7/0xb8

Fixes: 21844b0e32 ("PCI multi-seg: introduce notion of PCI segments")
Fixes: 8dc6738dbb ("Update radix-tree.[ch] from upstream Linux to gain RCU awareness")
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 1077a93425
master date: 2026-08-05 12:00:17 +0200

xen/common/radix-tree.c

@@ -32,12 +32,6 @@ struct radix_tree_path {
 #define RADIX_TREE_MAX_PATH (DIV_ROUND_UP(RADIX_TREE_INDEX_BITS, \
 					  RADIX_TREE_MAP_SHIFT))
 
-/*
- * The height_to_maxindex array needs to be one deeper than the maximum
- * path as height 0 holds only 1 entry.
- */
-static unsigned long height_to_maxindex[RADIX_TREE_MAX_PATH + 1] __read_mostly;
-
 static inline void *ptr_to_indirect(void *ptr)
 {
 	return (void *)((unsigned long)ptr | RADIX_TREE_INDIRECT_PTR);
@@ -80,7 +74,16 @@ static void radix_tree_node_free(struct radix_tree_node *node)
  */
 static inline unsigned long radix_tree_maxindex(unsigned int height)
 {
-	return height_to_maxindex[height];
+	unsigned int width = height * RADIX_TREE_MAP_SHIFT;
+	int shift = RADIX_TREE_INDEX_BITS - width;
+
+	if (shift < 0)
+		return ~0UL;
+
+	if (shift >= BITS_PER_LONG)
+		return 0UL;
+
+	return ~0UL >> shift;
 }
 
 /*
@@ -705,27 +708,3 @@ void radix_tree_init(struct radix_tree_root *root)
 {
 	*root = (struct radix_tree_root)RADIX_TREE_INIT();
 }
-
-static __init unsigned long __maxindex(unsigned int height)
-{
-	unsigned int width = height * RADIX_TREE_MAP_SHIFT;
-	int shift = RADIX_TREE_INDEX_BITS - width;
-
-	if (shift < 0)
-		return ~0UL;
-	if (shift >= BITS_PER_LONG)
-		return 0UL;
-	return ~0UL >> shift;
-}
-
-static int __init cf_check radix_tree_init_maxindex(void)
-{
-	unsigned int i;
-
-	for (i = 0; i < ARRAY_SIZE(height_to_maxindex); i++)
-		height_to_maxindex[i] = __maxindex(i);
-
-	return 0;
-}
-/* pre-SMP just so it runs before 'normal' initcalls */
-presmp_initcall(radix_tree_init_maxindex);