[v3] btrfs: fix check_shared for fiemap ioctl

Commit Message

Lu Fengqi June 8, 2016, 5:13 a.m. UTC

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 <[e-mail]>
Signed-off-by: Lu Fengqi <[e-mail]>
---
The caller is fiemap that called from an ioctl. Because it isn't on a
writeout path, so we temporarily use GFP_KERNEL in ref_root_alloc() and
ref_tree_add(). If we use ref_tree replace the existing backref structure
later, we can consider whether to use GFP_NOFS again.
---
 fs/btrfs/backref.c   | 362 +++++++++++++++++++++++++++++++++++++++++++++++++--
 fs/btrfs/extent_io.c |  18 ++-
 2 files changed, 370 insertions(+), 10 deletions(-)

Comments

Mark Fasheh June 8, 2016, 3:53 p.m. UTC | #1

On Wed, Jun 08, 2016 at 01:13:03PM +0800, Lu Fengqi wrote:
> 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 <[e-mail]>
> Signed-off-by: Lu Fengqi <[e-mail]>
> ---
> The caller is fiemap that called from an ioctl. Because it isn't on a
> writeout path, so we temporarily use GFP_KERNEL in ref_root_alloc() and
> ref_tree_add(). If we use ref_tree replace the existing backref structure
> later, we can consider whether to use GFP_NOFS again.

NACK.

You don't need to be on a writeout path to deadlock, you simply need to be
holding locks that the writeout path takes when you allocate. If the
allocator does writeout to free memory then you deadlock. Fiemap is locking 
down extents which may also get locked down when you allocate within those  
locks. See my e-mail here for details,

http://www.spinics.net/lists/linux-btrfs/msg55789.html
	--Mark

--
Mark Fasheh
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David Sterba June 9, 2016, 9:15 a.m. UTC | #2

On Wed, Jun 08, 2016 at 08:53:00AM -0700, Mark Fasheh wrote:
> On Wed, Jun 08, 2016 at 01:13:03PM +0800, Lu Fengqi wrote:
> > 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 <[e-mail]>
> > Signed-off-by: Lu Fengqi <[e-mail]>
> > ---
> > The caller is fiemap that called from an ioctl. Because it isn't on a
> > writeout path, so we temporarily use GFP_KERNEL in ref_root_alloc() and
> > ref_tree_add(). If we use ref_tree replace the existing backref structure
> > later, we can consider whether to use GFP_NOFS again.
> 
> NACK.
> 
> You don't need to be on a writeout path to deadlock, you simply need to be
> holding locks that the writeout path takes when you allocate. If the
> allocator does writeout to free memory then you deadlock. Fiemap is locking 
> down extents which may also get locked down when you allocate within those  
> locks. See my e-mail here for details,
> 
> http://www.spinics.net/lists/linux-btrfs/msg55789.html

There seems to be a code path that leads to a deadlock scenario, it
depends on the extent state of the file under fiemap. If there's a
delalloc range in the middle of the file, fiemap locks the whole range,
asks for memory that could trigger writeout, the delalloc range would
need to be written and requests locking the dealloc range again. There's
no direct locking but rather waiting for the extent LOCKED bit unset.

So, use GFP_NOFS now, though I still hope we can find a way to avoid
rb-tree and allocations alltogether.
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Lu Fengqi June 13, 2016, 1:19 a.m. UTC | #3

At 06/09/2016 05:15 PM, David Sterba wrote:
> On Wed, Jun 08, 2016 at 08:53:00AM -0700, Mark Fasheh wrote:
>> On Wed, Jun 08, 2016 at 01:13:03PM +0800, Lu Fengqi wrote:
>>> 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 <[e-mail]>
>>> Signed-off-by: Lu Fengqi <[e-mail]>
>>> ---
>>> The caller is fiemap that called from an ioctl. Because it isn't on a
>>> writeout path, so we temporarily use GFP_KERNEL in ref_root_alloc() and
>>> ref_tree_add(). If we use ref_tree replace the existing backref structure
>>> later, we can consider whether to use GFP_NOFS again.
>>
>> NACK.
>>
>> You don't need to be on a writeout path to deadlock, you simply need to be
>> holding locks that the writeout path takes when you allocate. If the
>> allocator does writeout to free memory then you deadlock. Fiemap is locking
>> down extents which may also get locked down when you allocate within those
>> locks. See my e-mail here for details,
>>
>> http://www.spinics.net/lists/linux-btrfs/msg55789.html
>
> There seems to be a code path that leads to a deadlock scenario, it
> depends on the extent state of the file under fiemap. If there's a
> delalloc range in the middle of the file, fiemap locks the whole range,
> asks for memory that could trigger writeout, the delalloc range would
> need to be written and requests locking the dealloc range again. There's
> no direct locking but rather waiting for the extent LOCKED bit unset.
>
> So, use GFP_NOFS now, though I still hope we can find a way to avoid
> rb-tree and allocations alltogether.
>
>

OK, until we find a way to avoid rb-tree and allocations alltogether, I 
will use GFP_NOFS.

Patch

diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index b90cd37..d02cfac 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -17,6 +17,7 @@ 
  */
 
 #include <linux/vmalloc.h>
+#include <linux/rbtree.h>
 #include "ctree.h"
 #include "disk-io.h"
 #include "backref.h"
@@ -34,6 +35,266 @@  struct extent_inode_elem {
 	struct extent_inode_elem *next;
 };
 
+/*
+ * ref_root is used as the root of the ref tree that hold a collection
+ * of unique references.
+ */
+struct ref_root {
+	struct rb_root rb_root;
+
+	/*
+	 * The unique_refs represents the number of ref_nodes with a positive
+	 * count stored in the tree. Even if a ref_node(the count is greater
+	 * than one) is added, the unique_refs will only increase one.
+	 */
+	unsigned int unique_refs;
+};
+
+/* ref_node is used to store a unique reference to the ref tree. */
+struct ref_node {
+	struct rb_node rb_node;
+
+	/* For NORMAL_REF, otherwise all these fields should be set to 0 */
+	u64 root_id;
+	u64 object_id;
+	u64 offset;
+
+	/* For SHARED_REF, otherwise parent field should be set to 0 */
+	u64 parent;
+
+	/* Ref to the ref_mod of btrfs_delayed_ref_node(delayed-ref.h) */
+	int ref_mod;
+};
+
+/* Dynamically allocate and initialize a ref_root */
+static struct ref_root *ref_root_alloc(void)
+{
+	struct ref_root *ref_tree;
+
+	ref_tree = kmalloc(sizeof(*ref_tree), GFP_KERNEL);
+	if (!ref_tree)
+		return NULL;
+
+	ref_tree->rb_root = RB_ROOT;
+	ref_tree->unique_refs = 0;
+
+	return ref_tree;
+}
+
+/* Free all nodes in the ref tree, and reinit ref_root */
+static void ref_root_fini(struct ref_root *ref_tree)
+{
+	struct ref_node *node;
+	struct rb_node *next;
+
+	while ((next = rb_first(&ref_tree->rb_root)) != NULL) {
+		node = rb_entry(next, struct ref_node, rb_node);
+		rb_erase(next, &ref_tree->rb_root);
+		kfree(node);
+	}
+
+	ref_tree->rb_root = RB_ROOT;
+	ref_tree->unique_refs = 0;
+}
+
+/* Free dynamically allocated ref_root */
+static void ref_root_free(struct ref_root *ref_tree)
+{
+	if (!ref_tree)
+		return;
+
+	ref_root_fini(ref_tree);
+	kfree(ref_tree);
+}
+
+/*
+ * Compare ref_node with (root_id, object_id, offset, parent)
+ *
+ * The function compares the two ref_node a and b. It returns an integer less
+ * than, equal to, or greater than zero , respectively, to be less than, to
+ * equal, or be greater than b.
+ */
+static int ref_node_cmp(struct ref_node *a, struct ref_node *b)
+{
+	if (a->root_id < b->root_id)
+		return -1;
+	else if (a->root_id > b->root_id)
+		return 1;
+
+	if (a->object_id < b->object_id)
+		return -1;
+	else if (a->object_id > b->object_id)
+		return 1;
+
+	if (a->offset < b->offset)
+		return -1;
+	else if (a->offset > b->offset)
+		return 1;
+
+	if (a->parent < b->parent)
+		return -1;
+	else if (a->parent > b->parent)
+		return 1;
+
+	return 0;
+}
+
+/*
+ * Search ref_node with (root_id, object_id, offset, parent) in the tree
+ *
+ * if found, the pointer of the ref_node will be returned;
+ * if not found, NULL will be returned and pos will point to the rb_node for
+ * insert, pos_parent will point to pos'parent for insert;
+*/
+static struct ref_node *__ref_tree_search(struct ref_root *ref_tree,
+					  struct rb_node ***pos,
+					  struct rb_node **pos_parent,
+					  u64 root_id, u64 object_id,
+					  u64 offset, u64 parent)
+{
+	struct ref_node *cur = NULL;
+	struct ref_node entry;
+	int ret;
+
+	entry.root_id = root_id;
+	entry.object_id = object_id;
+	entry.offset = offset;
+	entry.parent = parent;
+
+	*pos = &ref_tree->rb_root.rb_node;
+
+	while (**pos) {
+		*pos_parent = **pos;
+		cur = rb_entry(*pos_parent, struct ref_node, rb_node);
+
+		ret = ref_node_cmp(cur, &entry);
+		if (ret > 0)
+			*pos = &(**pos)->rb_left;
+		else if (ret < 0)
+			*pos = &(**pos)->rb_right;
+		else
+			return cur;
+	}
+
+	return NULL;
+}
+
+/*
+ * Insert a ref_node to the ref tree
+ * @pos used for specifiy the position to insert
+ * @pos_parent for specifiy pos's parent
+ *
+ * success, return 0;
+ * ref_node already exists, return -EEXIST;
+*/
+static int ref_tree_insert(struct ref_root *ref_tree, struct rb_node **pos,
+			   struct rb_node *pos_parent, struct ref_node *ins)
+{
+	struct rb_node **p = NULL;
+	struct rb_node *parent = NULL;
+	struct ref_node *cur = NULL;
+
+	if (!pos) {
+		cur = __ref_tree_search(ref_tree, &p, &parent, ins->root_id,
+					ins->object_id, ins->offset,
+					ins->parent);
+		if (cur)
+			return -EEXIST;
+	} else {
+		p = pos;
+		parent = pos_parent;
+	}
+
+	rb_link_node(&ins->rb_node, parent, p);
+	rb_insert_color(&ins->rb_node, &ref_tree->rb_root);
+
+	return 0;
+}
+
+/* Erase and free ref_node, caller should update ref_root->unique_refs */
+static void ref_tree_remove(struct ref_root *ref_tree, struct ref_node *node)
+{
+	rb_erase(&node->rb_node, &ref_tree->rb_root);
+	kfree(node);
+}
+
+/*
+ * Update ref_root->unique_refs
+ *
+ * Call __ref_tree_search
+ *	1. if ref_node doesn't exist, ref_tree_insert this node, and update
+ *	ref_root->unique_refs:
+ *		if ref_node->ref_mod > 0, ref_root->unique_refs++;
+ *		if ref_node->ref_mod < 0, do noting;
+ *
+ *	2. if ref_node is found, then get origin ref_node->ref_mod, and update
+ *	ref_node->ref_mod.
+ *		if ref_node->ref_mod is equal to 0,then call ref_tree_remove
+ *
+ *		according to origin_mod and new_mod, update ref_root->items
+ *		+----------------+--------------+-------------+
+ *		|		 |new_count <= 0|new_count > 0|
+ *		+----------------+--------------+-------------+
+ *		|origin_count < 0|       0      |      1      |
+ *		+----------------+--------------+-------------+
+ *		|origin_count > 0|      -1      |      0      |
+ *		+----------------+--------------+-------------+
+ *
+ * In case of allocation failure, -ENOMEM is returned and the ref_tree stays
+ * unaltered.
+ * Success, return 0
+ */
+static int ref_tree_add(struct ref_root *ref_tree, u64 root_id, u64 object_id,
+			u64 offset, u64 parent, int count)
+{
+	struct ref_node *node = NULL;
+	struct rb_node **pos = NULL;
+	struct rb_node *pos_parent = NULL;
+	int origin_count;
+	int ret;
+
+	if (!count)
+		return 0;
+
+	node = __ref_tree_search(ref_tree, &pos, &pos_parent, root_id,
+				 object_id, offset, parent);
+	if (node == NULL) {
+		node = kmalloc(sizeof(*node), GFP_KERNEL);
+		if (!node)
+			return -ENOMEM;
+
+		node->root_id = root_id;
+		node->object_id = object_id;
+		node->offset = offset;
+		node->parent = parent;
+		node->ref_mod = count;
+
+		ret = ref_tree_insert(ref_tree, pos, pos_parent, node);
+		ASSERT(!ret);
+		if (ret) {
+			kfree(node);
+			return ret;
+		}
+
+		ref_tree->unique_refs += node->ref_mod > 0 ? 1 : 0;
+
+		return 0;
+	}
+
+	origin_count = node->ref_mod;
+	node->ref_mod += count;
+
+	if (node->ref_mod > 0)
+		ref_tree->unique_refs += origin_count > 0 ? 0 : 1;
+	else if (node->ref_mod <= 0)
+		ref_tree->unique_refs += origin_count > 0 ? -1 : 0;
+
+	if (!node->ref_mod)
+		ref_tree_remove(ref_tree, node);
+
+	return 0;
+}
+
 static int check_extent_in_eb(struct btrfs_key *key, struct extent_buffer *eb,
 				struct btrfs_file_extent_item *fi,
 				u64 extent_item_pos,
@@ -703,6 +964,7 @@  static int __add_delayed_refs(struct btrfs_delayed_ref_head *head, u64 seq,
 static int __add_inline_refs(struct btrfs_fs_info *fs_info,
 			     struct btrfs_path *path, u64 bytenr,
 			     int *info_level, struct list_head *prefs,
+			     struct ref_root *ref_tree,
 			     u64 *total_refs, u64 inum)
 {
 	int ret = 0;
@@ -770,6 +1032,13 @@  static int __add_inline_refs(struct btrfs_fs_info *fs_info,
 			count = btrfs_shared_data_ref_count(leaf, sdref);
 			ret = __add_prelim_ref(prefs, 0, NULL, 0, offset,
 					       bytenr, count, GFP_NOFS);
+			if (ref_tree) {
+				if (!ret)
+					ret = ref_tree_add(ref_tree, 0, 0, 0,
+							   bytenr, count);
+				if (!ret && ref_tree->unique_refs > 1)
+					ret = BACKREF_FOUND_SHARED;
+			}
 			break;
 		}
 		case BTRFS_TREE_BLOCK_REF_KEY:
@@ -797,6 +1066,15 @@  static int __add_inline_refs(struct btrfs_fs_info *fs_info,
 			root = btrfs_extent_data_ref_root(leaf, dref);
 			ret = __add_prelim_ref(prefs, root, &key, 0, 0,
 					       bytenr, count, GFP_NOFS);
+			if (ref_tree) {
+				if (!ret)
+					ret = ref_tree_add(ref_tree, root,
+							   key.objectid,
+							   key.offset, 0,
+							   count);
+				if (!ret && ref_tree->unique_refs > 1)
+					ret = BACKREF_FOUND_SHARED;
+			}
 			break;
 		}
 		default:
@@ -815,7 +1093,8 @@  static int __add_inline_refs(struct btrfs_fs_info *fs_info,
  */
 static int __add_keyed_refs(struct btrfs_fs_info *fs_info,
 			    struct btrfs_path *path, u64 bytenr,
-			    int info_level, struct list_head *prefs, u64 inum)
+			    int info_level, struct list_head *prefs,
+			    struct ref_root *ref_tree, u64 inum)
 {
 	struct btrfs_root *extent_root = fs_info->extent_root;
 	int ret;
@@ -858,6 +1137,13 @@  static int __add_keyed_refs(struct btrfs_fs_info *fs_info,
 			count = btrfs_shared_data_ref_count(leaf, sdref);
 			ret = __add_prelim_ref(prefs, 0, NULL, 0, key.offset,
 						bytenr, count, GFP_NOFS);
+			if (ref_tree) {
+				if (!ret)
+					ret = ref_tree_add(ref_tree, 0, 0, 0,
+							   bytenr, count);
+				if (!ret && ref_tree->unique_refs > 1)
+					ret = BACKREF_FOUND_SHARED;
+			}
 			break;
 		}
 		case BTRFS_TREE_BLOCK_REF_KEY:
@@ -886,6 +1172,15 @@  static int __add_keyed_refs(struct btrfs_fs_info *fs_info,
 			root = btrfs_extent_data_ref_root(leaf, dref);
 			ret = __add_prelim_ref(prefs, root, &key, 0, 0,
 					       bytenr, count, GFP_NOFS);
+			if (ref_tree) {
+				if (!ret)
+					ret = ref_tree_add(ref_tree, root,
+							   key.objectid,
+							   key.offset, 0,
+							   count);
+				if (!ret && ref_tree->unique_refs > 1)
+					ret = BACKREF_FOUND_SHARED;
+			}
 			break;
 		}
 		default:
@@ -912,13 +1207,16 @@  static int __add_keyed_refs(struct btrfs_fs_info *fs_info,
  * commit root.
  * The special case is for qgroup to search roots in commit_transaction().
  *
+ * If check_shared is set to 1, any extent has more than one ref item, will
+ * be returned BACKREF_FOUND_SHARED immediately.
+ *
  * FIXME some caching might speed things up
  */
 static int find_parent_nodes(struct btrfs_trans_handle *trans,
 			     struct btrfs_fs_info *fs_info, u64 bytenr,
 			     u64 time_seq, struct ulist *refs,
 			     struct ulist *roots, const u64 *extent_item_pos,
-			     u64 root_objectid, u64 inum)
+			     u64 root_objectid, u64 inum, int check_shared)
 {
 	struct btrfs_key key;
 	struct btrfs_path *path;
@@ -930,6 +1228,7 @@  static int find_parent_nodes(struct btrfs_trans_handle *trans,
 	struct list_head prefs;
 	struct __prelim_ref *ref;
 	struct extent_inode_elem *eie = NULL;
+	struct ref_root *ref_tree = NULL;
 	u64 total_refs = 0;
 
 	INIT_LIST_HEAD(&prefs);
@@ -961,6 +1260,18 @@  static int find_parent_nodes(struct btrfs_trans_handle *trans,
 again:
 	head = NULL;
 
+	if (check_shared) {
+		if (!ref_tree) {
+			ref_tree = ref_root_alloc();
+			if (!ref_tree) {
+				ret = -ENOMEM;
+				goto out;
+			}
+		} else {
+			ref_root_fini(ref_tree);
+		}
+	}
+
 	ret = btrfs_search_slot(trans, fs_info->extent_root, &key, path, 0, 0);
 	if (ret < 0)
 		goto out;
@@ -1005,6 +1316,36 @@  again:
 		} else {
 			spin_unlock(&delayed_refs->lock);
 		}
+
+		if (check_shared && !list_empty(&prefs_delayed)) {
+			/*
+			 * Add all delay_ref to the ref_tree and check if there
+			 * are multiple ref items added.
+			 */
+			list_for_each_entry(ref, &prefs_delayed, list) {
+				if (ref->key_for_search.type) {
+					ret = ref_tree_add(ref_tree,
+						ref->root_id,
+						ref->key_for_search.objectid,
+						ref->key_for_search.offset,
+						0, ref->count);
+					if (ret)
+						goto out;
+				} else {
+					ret = ref_tree_add(ref_tree, 0, 0, 0,
+						     ref->parent, ref->count);
+					if (ret)
+						goto out;
+				}
+
+			}
+
+			if (ref_tree->unique_refs > 1) {
+				ret = BACKREF_FOUND_SHARED;
+				goto out;
+			}
+
+		}
 	}
 
 	if (path->slots[0]) {
@@ -1020,11 +1361,13 @@  again:
 		     key.type == BTRFS_METADATA_ITEM_KEY)) {
 			ret = __add_inline_refs(fs_info, path, bytenr,
 						&info_level, &prefs,
-						&total_refs, inum);
+						ref_tree, &total_refs,
+						inum);
 			if (ret)
 				goto out;
 			ret = __add_keyed_refs(fs_info, path, bytenr,
-					       info_level, &prefs, inum);
+					       info_level, &prefs,
+					       ref_tree, inum);
 			if (ret)
 				goto out;
 		}
@@ -1109,6 +1452,7 @@  again:
 
 out:
 	btrfs_free_path(path);
+	ref_root_free(ref_tree);
 	while (!list_empty(&prefs)) {
 		ref = list_first_entry(&prefs, struct __prelim_ref, list);
 		list_del(&ref->list);
@@ -1162,8 +1506,8 @@  static int btrfs_find_all_leafs(struct btrfs_trans_handle *trans,
 	if (!*leafs)
 		return -ENOMEM;
 
-	ret = find_parent_nodes(trans, fs_info, bytenr,
-				time_seq, *leafs, NULL, extent_item_pos, 0, 0);
+	ret = find_parent_nodes(trans, fs_info, bytenr, time_seq,
+				*leafs, NULL, extent_item_pos, 0, 0, 0);
 	if (ret < 0 && ret != -ENOENT) {
 		free_leaf_list(*leafs);
 		return ret;
@@ -1205,8 +1549,8 @@  static int __btrfs_find_all_roots(struct btrfs_trans_handle *trans,
 
 	ULIST_ITER_INIT(&uiter);
 	while (1) {
-		ret = find_parent_nodes(trans, fs_info, bytenr,
-					time_seq, tmp, *roots, NULL, 0, 0);
+		ret = find_parent_nodes(trans, fs_info, bytenr, time_seq,
+					tmp, *roots, NULL, 0, 0, 0);
 		if (ret < 0 && ret != -ENOENT) {
 			ulist_free(tmp);
 			ulist_free(*roots);
@@ -1276,7 +1620,7 @@  int btrfs_check_shared(struct btrfs_trans_handle *trans,
 	ULIST_ITER_INIT(&uiter);
 	while (1) {
 		ret = find_parent_nodes(trans, fs_info, bytenr, elem.seq, tmp,
-					roots, NULL, root_objectid, inum);
+					roots, NULL, root_objectid, inum, 1);
 		if (ret == BACKREF_FOUND_SHARED) {
 			/* this is the only condition under which we return 1 */
 			ret = 1;
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 2e7c97a..31a174a 100644
--- a/fs/btrfs/extent_io.c
+++ b/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;
@@ -4498,21 +4499,36 @@  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
 			 * flag FIEMAP_EXTENT_SHARED.  If fi_extents_max == 0
 			 * 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)