Btrfs: fix data allocation hint start

Sometimes our start allocation hint when we cow a file can be either
EXTENT_HOLE or some other such place holder, which is not optimal.  So if we
find that our em->block_start is one of these special values, check to see
where the first block of the inode is stored, and use that as a hint.  If that
block is also a special value, just fallback on a hint of 0 and let the
allocator figure out a good place to put the data.

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 78139ef..d8393dd 100644 (file)
--- 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) {
        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);
        }
        read_unlock(&BTRFS_I(inode)->extent_tree.lock);
        btrfs_drop_extent_cache(inode, start, start + num_bytes - 1, 0);