mm: khugepaged: fix radix tree node leak in shmem collapse error path

The radix tree counts valid entries in each tree node.  Entries stored
in the tree cannot be removed by simpling storing NULL in the slot or
the internal counters will be off and the node never gets freed again.

When collapsing a shmem page fails, restore the holes that were filled
with radix_tree_insert() with a proper radix tree deletion.

Fixes: f3f0e1d2 ("khugepaged: add support of collapse for tmpfs/shmem pages")
Link: http://lkml.kernel.org/r/20161117191138.22769-3-hannes@cmpxchg.orgSigned-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Jan Kara <jack@suse.cz>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox <mawilcox@linuxonhyperv.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/khugepaged.c

@@ -1525,9 +1525,11 @@ static void collapse_shmem(struct mm_struct *mm,
 			if (!page || iter.index < page->index) {
 				if (!nr_none)
 					break;
-				/* Put holes back where they were */
-				radix_tree_replace_slot(slot, NULL);
 				nr_none--;
+				/* Put holes back where they were */
+				radix_tree_delete(&mapping->page_tree,
+						  iter.index);
+				slot = radix_tree_iter_next(&iter);
 				continue;
 			}