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, 3 insertions, 4 deletions

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 7434a63cac94..e32389a97030 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1446,7 +1446,7 @@ static void collapse_shmem(struct mm_struct *mm,
 		radix_tree_replace_slot(&mapping->page_tree, slot,
 				new_page + (index % HPAGE_PMD_NR));
 
-		slot = radix_tree_iter_next(&iter);
+		slot = radix_tree_iter_resume(slot, &iter);
 		index++;
 		continue;
 out_lru:
@@ -1546,7 +1546,6 @@ tree_unlocked:
 				/* Put holes back where they were */
 				radix_tree_delete(&mapping->page_tree,
 						  iter.index);
-				slot = radix_tree_iter_next(&iter);
 				continue;
 			}
 
@@ -1557,11 +1556,11 @@ tree_unlocked:
 			page_ref_unfreeze(page, 2);
 			radix_tree_replace_slot(&mapping->page_tree,
 						slot, page);
+			slot = radix_tree_iter_resume(slot, &iter);
 			spin_unlock_irq(&mapping->tree_lock);
 			putback_lru_page(page);
 			unlock_page(page);
 			spin_lock_irq(&mapping->tree_lock);
-			slot = radix_tree_iter_next(&iter);
 		}
 		VM_BUG_ON(nr_none);
 		spin_unlock_irq(&mapping->tree_lock);
@@ -1641,8 +1640,8 @@ static void khugepaged_scan_shmem(struct mm_struct *mm,
 		present++;
 
 		if (need_resched()) {
+			slot = radix_tree_iter_resume(slot, &iter);
 			cond_resched_rcu();
-			slot = radix_tree_iter_next(&iter);
 		}
 	}
 	rcu_read_unlock();