Btrfs: make fiemap not blow when you have lots of snapshots

We have been iterating all references for each extent we have in a file when we
do fiemap to see if it is shared.  This is fine when you have a few clones or a
few snapshots, but when you have 5k snapshots suddenly fiemap just sits there
and stares at you.  So add btrfs_check_shared which will use the backref walking
code but will short circuit as soon as it finds a root or inode that doesn't
match the one we currently have.  This makes fiemap on my testbox go from
looking at me blankly for a day to spitting out actual output in a reasonable
amount of time.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/btrfs/backref.h b/fs/btrfs/backref.h
index 86fc20fec282..2a1ac6bfc724 100644
--- a/fs/btrfs/backref.h
+++ b/fs/btrfs/backref.h
@@ -71,6 +71,9 @@ int btrfs_find_one_extref(struct btrfs_root *root, u64 inode_objectid,
                           u64 start_off, struct btrfs_path *path,
                           struct btrfs_inode_extref **ret_extref,
                           u64 *found_off);
+int btrfs_check_shared(struct btrfs_trans_handle *trans,
+                       struct btrfs_fs_info *fs_info, u64 root_objectid,
+                       u64 inum, u64 bytenr);
 
 int __init btrfs_prelim_ref_init(void);
 void btrfs_prelim_ref_exit(void);