btrfs: btrfs_check_shared should manage its own transaction

Commit afce772e87c3 ("btrfs: fix check_shared for fiemap ioctl") added
transaction semantics around calls to btrfs_check_shared() in order to
provide accurate accounting of delayed refs. The transaction management
should be done inside btrfs_check_shared(), so that callers do not need
to manage transactions individually.

Signed-off-by: Edmund Nadolski <enadolski@suse.com>
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/backref.c

@@ -1580,20 +1580,21 @@ int btrfs_find_all_roots(struct btrfs_trans_handle *trans,
 /**
  * btrfs_check_shared - tell us whether an extent is shared
  *
- * @trans: optional trans handle
- *
  * btrfs_check_shared uses the backref walking code but will short
  * circuit as soon as it finds a root or inode that doesn't match the
  * one passed in. This provides a significant performance benefit for
  * callers (such as fiemap) which want to know whether the extent is
  * shared but do not need a ref count.
  *
+ * This attempts to allocate a transaction in order to account for
+ * delayed refs, but continues on even when the alloc fails.
+ *
  * Return: 0 if extent is not shared, 1 if it is shared, < 0 on error.
  */
-int btrfs_check_shared(struct btrfs_trans_handle *trans,
-		       struct btrfs_fs_info *fs_info, u64 root_objectid,
-		       u64 inum, u64 bytenr)
+int btrfs_check_shared(struct btrfs_root *root, u64 inum, u64 bytenr)
 {
+	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct btrfs_trans_handle *trans;
 	struct ulist *tmp = NULL;
 	struct ulist *roots = NULL;
 	struct ulist_iterator uiter;
@@ -1609,14 +1610,18 @@ int btrfs_check_shared(struct btrfs_trans_handle *trans,
 		return -ENOMEM;
 	}
 
-	if (trans)
-		btrfs_get_tree_mod_seq(fs_info, &elem);
-	else
+	trans = btrfs_join_transaction(root);
+	if (IS_ERR(trans)) {
+		trans = NULL;
 		down_read(&fs_info->commit_root_sem);
+	} else {
+		btrfs_get_tree_mod_seq(fs_info, &elem);
+	}
+
 	ULIST_ITER_INIT(&uiter);
 	while (1) {
 		ret = find_parent_nodes(trans, fs_info, bytenr, elem.seq, tmp,
-					roots, NULL, root_objectid, inum, 1);
+					roots, NULL, root->objectid, inum, 1);
 		if (ret == BACKREF_FOUND_SHARED) {
 			/* this is the only condition under which we return 1 */
 			ret = 1;
@@ -1631,10 +1636,13 @@ int btrfs_check_shared(struct btrfs_trans_handle *trans,
 		bytenr = node->val;
 		cond_resched();
 	}
-	if (trans)
+
+	if (trans) {
 		btrfs_put_tree_mod_seq(fs_info, &elem);
-	else
+		btrfs_end_transaction(trans);
+	} else {
 		up_read(&fs_info->commit_root_sem);
+	}
 	ulist_free(tmp);
 	ulist_free(roots);
 	return ret;

fs/btrfs/backref.h

@@ -68,9 +68,7 @@ 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 btrfs_check_shared(struct btrfs_root *root, u64 inum, u64 bytenr);
 
 int __init btrfs_prelim_ref_init(void);
 void btrfs_prelim_ref_exit(void);

fs/btrfs/extent_io.c

@@ -20,7 +20,6 @@
 #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;
@@ -4606,24 +4605,11 @@ 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
@@ -4631,11 +4617,9 @@ 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(trans, root->fs_info,
-					root->objectid,
-					btrfs_ino(BTRFS_I(inode)), bytenr);
-			if (trans)
-				btrfs_end_transaction(trans);
+			ret = btrfs_check_shared(root,
+						 btrfs_ino(BTRFS_I(inode)),
+						 bytenr);
 			if (ret < 0)
 				goto out_free;
 			if (ret)