btrfs: fix check_shared for fiemap ioctl

Only in the case of different root_id or different object_id, check_shared
identified extent as the shared. However, If a extent was referred by
different offset of same file, it should also be identified as shared.
In addition, check_shared's loop scale is at least n^3, so if a extent
has too many references, even causes soft hang up.

First, add all delayed_ref to the ref_tree and calculate the unqiue_refs,
if the unique_refs is greater than one, return BACKREF_FOUND_SHARED.
Then individually add the on-disk reference(inline/keyed) to the ref_tree
and calculate the unique_refs of the ref_tree to check if the unique_refs
is greater than one.Because once there are two references to return
SHARED, so the time complexity is close to the constant.
Reported-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -20,6 +20,7 @@
 #include "locking.h"
 #include "rcu-string.h"
 #include "backref.h"
+#include "transaction.h"
 
 static struct kmem_cache *extent_state_cache;
 static struct kmem_cache *extent_buffer_cache;
@@ -4487,11 +4488,24 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 			flags |= (FIEMAP_EXTENT_DELALLOC |
 				  FIEMAP_EXTENT_UNKNOWN);
 		} else if (fieinfo->fi_extents_max) {
+			struct btrfs_trans_handle *trans;
+
 			u64 bytenr = em->block_start -
 				(em->start - em->orig_start);
 
 			disko = em->block_start + offset_in_extent;
 
+			/*
+			 * We need a trans handle to get delayed refs
+			 */
+			trans = btrfs_join_transaction(root);
+			/*
+			 * It's OK if we can't start a trans we can still check
+			 * from commit_root
+			 */
+			if (IS_ERR(trans))
+				trans = NULL;
+
 			/*
 			 * As btrfs supports shared space, this information
 			 * can be exported to userspace tools via
@@ -4499,9 +4513,11 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 			 * then we're just getting a count and we can skip the
 			 * lookup stuff.
 			 */
-			ret = btrfs_check_shared(NULL, root->fs_info,
+			ret = btrfs_check_shared(trans, root->fs_info,
 						 root->objectid,
 						 btrfs_ino(inode), bytenr);
+			if (trans)
+				btrfs_end_transaction(trans, root);
 			if (ret < 0)
 				goto out_free;
 			if (ret)