1 files changed, 16 insertions, 2 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 78139efe41fc..d8393ddc72a3 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -743,8 +743,22 @@ static noinline int cow_file_range(struct inode *inode,
         em = search_extent_mapping(&BTRFS_I(inode)->extent_tree,
                                    start, num_bytes);
         if (em) {
-                alloc_hint = em->block_start;
-                free_extent_map(em);
+                /*
+                 * if block start isn't an actual block number then find the
+                 * first block in this inode and use that as a hint.  If that
+                 * block is also bogus then just don't worry about it.
+                 */
+                if (em->block_start >= EXTENT_MAP_LAST_BYTE) {
+                        free_extent_map(em);
+                        em = search_extent_mapping(em_tree, 0, 0);
+                        if (em && em->block_start < EXTENT_MAP_LAST_BYTE)
+                                alloc_hint = em->block_start;
+                        if (em)
+                                free_extent_map(em);
+                } else {
+                        alloc_hint = em->block_start;
+                        free_extent_map(em);
+                }
         }
         read_unlock(&BTRFS_I(inode)->extent_tree.lock);
         btrfs_drop_extent_cache(inode, start, start + num_bytes - 1, 0);