Btrfs: Support reading/writing on disk free ino cache

This is similar to block group caching.

We dedicate a special inode in fs tree to save free ino cache.

At the very first time we create/delete a file after mount, the free ino
cache will be loaded from disk into memory. When the fs tree is commited,
the cache will be written back to disk.

To keep compatibility, we check the root generation against the generation
of the special inode when loading the cache, so the loading will fail
if the btrfs filesystem was mounted in an older kernel before.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index adec228..b78d3ab 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -745,6 +745,15 @@
 	return alloc_hint;
 }
 
+static inline bool is_free_space_inode(struct btrfs_root *root,
+				       struct inode *inode)
+{
+	if (root == root->fs_info->tree_root ||
+	    BTRFS_I(inode)->location.objectid == BTRFS_FREE_INO_OBJECTID)
+		return true;
+	return false;
+}
+
 /*
  * when extent_io.c finds a delayed allocation range in the file,
  * the call backs end up in this code.  The basic idea is to
@@ -777,7 +786,7 @@
 	struct extent_map_tree *em_tree = &BTRFS_I(inode)->extent_tree;
 	int ret = 0;
 
-	BUG_ON(root == root->fs_info->tree_root);
+	BUG_ON(is_free_space_inode(root, inode));
 	trans = btrfs_join_transaction(root, 1);
 	BUG_ON(IS_ERR(trans));
 	btrfs_set_trans_block_group(trans, inode);
@@ -1048,17 +1057,18 @@
 	int type;
 	int nocow;
 	int check_prev = 1;
-	bool nolock = false;
+	bool nolock;
 	u64 ino = btrfs_ino(inode);
 
 	path = btrfs_alloc_path();
 	BUG_ON(!path);
-	if (root == root->fs_info->tree_root) {
-		nolock = true;
+
+	nolock = is_free_space_inode(root, inode);
+
+	if (nolock)
 		trans = btrfs_join_transaction_nolock(root, 1);
-	} else {
+	else
 		trans = btrfs_join_transaction(root, 1);
-	}
 	BUG_ON(IS_ERR(trans));
 
 	cow_start = (u64)-1;
@@ -1316,8 +1326,7 @@
 	if (!(state->state & EXTENT_DELALLOC) && (*bits & EXTENT_DELALLOC)) {
 		struct btrfs_root *root = BTRFS_I(inode)->root;
 		u64 len = state->end + 1 - state->start;
-		int do_list = (root->root_key.objectid !=
-			       BTRFS_ROOT_TREE_OBJECTID);
+		bool do_list = !is_free_space_inode(root, inode);
 
 		if (*bits & EXTENT_FIRST_DELALLOC)
 			*bits &= ~EXTENT_FIRST_DELALLOC;
@@ -1350,8 +1359,7 @@
 	if ((state->state & EXTENT_DELALLOC) && (*bits & EXTENT_DELALLOC)) {
 		struct btrfs_root *root = BTRFS_I(inode)->root;
 		u64 len = state->end + 1 - state->start;
-		int do_list = (root->root_key.objectid !=
-			       BTRFS_ROOT_TREE_OBJECTID);
+		bool do_list = !is_free_space_inode(root, inode);
 
 		if (*bits & EXTENT_FIRST_DELALLOC)
 			*bits &= ~EXTENT_FIRST_DELALLOC;
@@ -1458,7 +1466,7 @@
 
 	skip_sum = BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM;
 
-	if (root == root->fs_info->tree_root)
+	if (is_free_space_inode(root, inode))
 		ret = btrfs_bio_wq_end_io(root->fs_info, bio, 2);
 	else
 		ret = btrfs_bio_wq_end_io(root->fs_info, bio, 0);
@@ -1701,7 +1709,7 @@
 	struct extent_state *cached_state = NULL;
 	int compress_type = 0;
 	int ret;
-	bool nolock = false;
+	bool nolock;
 
 	ret = btrfs_dec_test_ordered_pending(inode, &ordered_extent, start,
 					     end - start + 1);
@@ -1709,7 +1717,7 @@
 		return 0;
 	BUG_ON(!ordered_extent);
 
-	nolock = (root == root->fs_info->tree_root);
+	nolock = is_free_space_inode(root, inode);
 
 	if (test_bit(BTRFS_ORDERED_NOCOW, &ordered_extent->flags)) {
 		BUG_ON(!list_empty(&ordered_extent->list));
@@ -3473,7 +3481,9 @@
 
 		if (path->slots[0] == 0 ||
 		    path->slots[0] != pending_del_slot) {
-			if (root->ref_cows) {
+			if (root->ref_cows &&
+			    BTRFS_I(inode)->location.objectid !=
+						BTRFS_FREE_INO_OBJECTID) {
 				err = -EAGAIN;
 				goto out;
 			}
@@ -3765,7 +3775,7 @@
 
 	truncate_inode_pages(&inode->i_data, 0);
 	if (inode->i_nlink && (btrfs_root_refs(&root->root_item) != 0 ||
-			       root == root->fs_info->tree_root))
+			       is_free_space_inode(root, inode)))
 		goto no_delete;
 
 	if (is_bad_inode(inode)) {
@@ -4382,7 +4392,8 @@
 		return 0;
 
 	smp_mb();
-	nolock = (root->fs_info->closing && root == root->fs_info->tree_root);
+	if (root->fs_info->closing && is_free_space_inode(root, inode))
+		nolock = true;
 
 	if (wbc->sync_mode == WB_SYNC_ALL) {
 		if (nolock)
@@ -6900,7 +6911,7 @@
 	struct btrfs_root *root = BTRFS_I(inode)->root;
 
 	if (btrfs_root_refs(&root->root_item) == 0 &&
-	    root != root->fs_info->tree_root)
+	    !is_free_space_inode(root, inode))
 		return 1;
 	else
 		return generic_drop_inode(inode);