radix_tree: add support for multi-order entries

With huge pages, it is convenient to have the radix tree be able to
return an entry that covers multiple indices.  Previous attempts to deal
with the problem have involved inserting N duplicate entries, which is a
waste of memory and leads to problems trying to handle aliased tags, or
probing the tree multiple times to find alternative entries which might
cover the requested index.

This approach inserts one canonical entry into the tree for a given
range of indices, and may also insert other entries in order to ensure
that lookups find the canonical entry.

This solution only tolerates inserting powers of two that are greater
than the fanout of the tree.  If we wish to expand the radix tree's
abilities to support large-ish pages that is less than the fanout at the
penultimate level of the tree, then we would need to add one more step
in lookup to ensure that any sibling nodes in the final level of the
tree are dereferenced and we return the canonical entry that they
reference.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@linux.intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 2 deletions

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index 39598b9cf1d9..b211f145c811 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -271,8 +271,15 @@ static inline void radix_tree_replace_slot(void **pslot, void *item)
 }
 
 int __radix_tree_create(struct radix_tree_root *root, unsigned long index,
-			struct radix_tree_node **nodep, void ***slotp);
-int radix_tree_insert(struct radix_tree_root *, unsigned long, void *);
+			unsigned order, struct radix_tree_node **nodep,
+			void ***slotp);
+int __radix_tree_insert(struct radix_tree_root *, unsigned long index,
+			unsigned order, void *);
+static inline int radix_tree_insert(struct radix_tree_root *root,
+			unsigned long index, void *entry)
+{
+	return __radix_tree_insert(root, index, 0, entry);
+}
 void *__radix_tree_lookup(struct radix_tree_root *root, unsigned long index,
 			  struct radix_tree_node **nodep, void ***slotp);
 void *radix_tree_lookup(struct radix_tree_root *, unsigned long);