Btrfs: fix a bug of snapshot-aware defrag to make it work on partial extents

For partial extents, snapshot-aware defrag does not work as expected,
since
a) we use the wrong logical offset to search for parents, which should be
   disk_bytenr + extent_offset, not just disk_bytenr,
b) 'offset' returned by the backref walking just refers to key.offset, not
   the 'offset' stored in btrfs_extent_data_ref which is
   (key.offset - extent_offset).

The reproducer:
$ mkfs.btrfs sda
$ mount sda /mnt
$ btrfs sub create /mnt/sub
$ for i in `seq 5 -1 1`; do dd if=/dev/zero of=/mnt/sub/foo bs=5k count=1 seek=$i conv=notrunc oflag=sync; done
$ btrfs sub snap /mnt/sub /mnt/snap1
$ btrfs sub snap /mnt/sub /mnt/snap2
$ sync; btrfs filesystem defrag /mnt/sub/foo;
$ umount /mnt
$ btrfs-debug-tree sda (Here we can check whether the defrag operation is snapshot-awared.

This addresses the above two problems.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

1 files changed, 12 insertions, 4 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 0fd7647c8932..c72033ee6017 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -2166,16 +2166,23 @@ static noinline int record_one_backref(u64 inum, u64 offset, u64 root_id,
                 if (btrfs_file_extent_disk_bytenr(leaf, extent) != old->bytenr)
                         continue;
 
-                extent_offset = btrfs_file_extent_offset(leaf, extent);
-                if (key.offset - extent_offset != offset)
+                /*
+                 * 'offset' refers to the exact key.offset,
+                 * NOT the 'offset' field in btrfs_extent_data_ref, ie.
+                 * (key.offset - extent_offset).
+                 */
+                if (key.offset != offset)
                         continue;
 
+                extent_offset = btrfs_file_extent_offset(leaf, extent);
                 num_bytes = btrfs_file_extent_num_bytes(leaf, extent);
+
                 if (extent_offset >= old->extent_offset + old->offset +
                     old->len || extent_offset + num_bytes <=
                     old->extent_offset + old->offset)
                         continue;
 
+                ret = 0;
                 break;
         }
 
@@ -2187,7 +2194,7 @@ static noinline int record_one_backref(u64 inum, u64 offset, u64 root_id,
 
         backref->root_id = root_id;
         backref->inum = inum;
-        backref->file_pos = offset + extent_offset;
+        backref->file_pos = offset;
         backref->num_bytes = num_bytes;
         backref->extent_offset = extent_offset;
         backref->generation = btrfs_file_extent_generation(leaf, extent);
@@ -2210,7 +2217,8 @@ static noinline bool record_extent_backrefs(struct btrfs_path *path,
         new->path = path;
 
         list_for_each_entry_safe(old, tmp, &new->head, list) {
-                ret = iterate_inodes_from_logical(old->bytenr, fs_info,
+                ret = iterate_inodes_from_logical(old->bytenr +
+                                                  old->extent_offset, fs_info,
                                                   path, record_one_backref,
                                                   old);
                 BUG_ON(ret < 0 && ret != -ENOENT);