radix-tree: improve multiorder iterators

This fixes several interlinked problems with the iterators in the
presence of multiorder entries.

1. radix_tree_iter_next() would only advance by one slot, which would
   result in the iterators returning the same entry more than once if
   there were sibling entries.

2. radix_tree_next_slot() could return an internal pointer instead of
   a user pointer if a tagged multiorder entry was immediately followed by
   an entry of lower order.

3. radix_tree_next_slot() expanded to a lot more code than it used to
   when multiorder support was compiled in.  And I wasn't comfortable with
   entry_to_node() being in a header file.

Fixing radix_tree_iter_next() for the presence of sibling entries
necessarily involves examining the contents of the radix tree, so we now
need to pass 'slot' to radix_tree_iter_next(), and we need to change the
calling convention so it is called *before* dropping the lock which
protects the tree.  Also rename it to radix_tree_iter_resume(), as some
people thought it was necessary to call radix_tree_iter_next() each time
around the loop.

radix_tree_next_slot() becomes closer to how it looked before multiorder
support was introduced.  It only checks to see if the next entry in the
chunk is a sibling entry or a pointer to a node; this should be rare
enough that handling this case out of line is not a performance impact
(and such impact is amortised by the fact that the entry we just
processed was a multiorder entry).  Also, radix_tree_next_slot() used to
force a new chunk lookup for untagged entries, which is more expensive
than the out of line sibling entry skipping.

Link: http://lkml.kernel.org/r/1480369871-5271-55-git-send-email-mawilcox@linuxonhyperv.com
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Tested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 4 deletions

diff --git a/tools/testing/radix-tree/regression3.c b/tools/testing/radix-tree/regression3.c
index 1f06ed73d0a8..b594841fae85 100644
--- a/tools/testing/radix-tree/regression3.c
+++ b/tools/testing/radix-tree/regression3.c
@@ -5,7 +5,7 @@
  * In following radix_tree_next_slot current chunk size becomes zero.
  * This isn't checked and it tries to dereference null pointer in slot.
  *
- * Helper radix_tree_iter_next reset slot to NULL and next_index to index + 1,
+ * Helper radix_tree_iter_resume reset slot to NULL and next_index to index + 1,
  * for tagger iteraction it also must reset cached tags in iterator to abort
  * next radix_tree_next_slot and go to slow-path into radix_tree_next_chunk.
  *
@@ -88,7 +88,7 @@ void regression3_test(void)
                 printf("slot %ld %p\n", iter.index, *slot);
                 if (!iter.index) {
                         printf("next at %ld\n", iter.index);
-                        slot = radix_tree_iter_next(&iter);
+                        slot = radix_tree_iter_resume(slot, &iter);
                 }
         }
 
@@ -96,7 +96,7 @@ void regression3_test(void)
                 printf("contig %ld %p\n", iter.index, *slot);
                 if (!iter.index) {
                         printf("next at %ld\n", iter.index);
-                        slot = radix_tree_iter_next(&iter);
+                        slot = radix_tree_iter_resume(slot, &iter);
                 }
         }
 
@@ -106,7 +106,7 @@ void regression3_test(void)
                 printf("tagged %ld %p\n", iter.index, *slot);
                 if (!iter.index) {
                         printf("next at %ld\n", iter.index);
-                        slot = radix_tree_iter_next(&iter);
+                        slot = radix_tree_iter_resume(slot, &iter);
                 }
         }