[PATCH] radix-tree: drop radix_tree_init_maxindex()

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Fixes: 21844b0e32e7 ("PCI multi-seg: introduce notion of PCI segments")
Fixes: 8dc6738dbb3c ("Update radix-tree.[ch] from upstream Linux to gain RCU 
awareness")
Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Two Fixes: tags because the use of a pre-SMP initcall was clearly setting
up a trap for later code to fall into.

I know for certain that I've seen logs of Xen running on multi-segment
systems. I can't quite explain how that ended up working.

--- a/xen/common/radix-tree.c
+++ b/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
  */
 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_r
 {
        *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);