Btrfs: update space balancing code

This patch updates the space balancing code to utilize the new
backref format.  Before, btrfs-vol -b would break any COW links
on data blocks or metadata.  This was slow and caused the amount
of space used to explode if a large number of snapshots were present.

The new code can keeps the sharing of all data extents and
most of the tree blocks.

To maintain the sharing of data extents, the space balance code uses
a seperate inode hold data extent pointers, then updates the references
to point to the new location.

To maintain the sharing of tree blocks, the space balance code uses
reloc trees to relocate tree blocks in reference counted roots.
There is one reloc tree for each subvol, and all reloc trees share
same root key objectid. Reloc trees are snapshots of the latest
committed roots of subvols (root->commit_root).

To relocate a tree block referenced by a subvol, there are two steps.
COW the block through subvol's reloc tree, then update block pointer in
the subvol to point to the new block. Since all reloc trees share
same root key objectid, doing special handing for tree blocks
owned by them is easy. Once a tree block has been COWed in one
reloc tree, we can use the resulting new block directly when the
same block is required to COW again through other reloc trees.
In this way, relocated tree blocks are shared between reloc trees,
so they are also shared between subvols.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 153 insertions, 2 deletions

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index f9cd40967d04..50e81f43e6d4 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -179,7 +179,6 @@ int noinline __btrfs_cow_block(struct btrfs_trans_handle *trans,
         struct extent_buffer *cow;
         u32 nritems;
         int ret = 0;
-        int different_trans = 0;
         int level;
         int unlock_orig = 0;
 
@@ -233,13 +232,33 @@ int noinline __btrfs_cow_block(struct btrfs_trans_handle *trans,
         WARN_ON(btrfs_header_generation(buf) > trans->transid);
         if (btrfs_header_generation(buf) != trans->transid) {
                 u32 nr_extents;
-                different_trans = 1;
                 ret = btrfs_inc_ref(trans, root, buf, cow, &nr_extents);
                 if (ret)
                         return ret;
 
                 ret = btrfs_cache_ref(trans, root, buf, nr_extents);
                 WARN_ON(ret);
+        } else if (btrfs_header_owner(buf) == BTRFS_TREE_RELOC_OBJECTID) {
+                /*
+                 * There are only two places that can drop reference to
+                 * tree blocks owned by living reloc trees, one is here,
+                 * the other place is btrfs_merge_path. In both places,
+                 * we check reference count while tree block is locked.
+                 * Furthermore, if reference count is one, it won't get
+                 * increased by someone else.
+                 */
+                u32 refs;
+                ret = btrfs_lookup_extent_ref(trans, root, buf->start,
+                                              buf->len, &refs);
+                BUG_ON(ret);
+                if (refs == 1) {
+                        ret = btrfs_update_ref(trans, root, buf, cow,
+                                               0, nritems);
+                        clean_tree_block(trans, root, buf);
+                } else {
+                        ret = btrfs_inc_ref(trans, root, buf, cow, NULL);
+                }
+                BUG_ON(ret);
         } else {
                 ret = btrfs_update_ref(trans, root, buf, cow, 0, nritems);
                 if (ret)
@@ -247,6 +266,14 @@ int noinline __btrfs_cow_block(struct btrfs_trans_handle *trans,
                 clean_tree_block(trans, root, buf);
         }
 
+        if (root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID) {
+                ret = btrfs_add_reloc_mapping(root, buf->start,
+                                              buf->len, cow->start);
+                BUG_ON(ret);
+                ret = btrfs_reloc_tree_cache_ref(trans, root, cow, buf->start);
+                WARN_ON(ret);
+        }
+
         if (buf == root->node) {
                 WARN_ON(parent && parent != buf);
 
@@ -1466,6 +1493,130 @@ done:
         return ret;
 }
 
+int btrfs_merge_path(struct btrfs_trans_handle *trans,
+                     struct btrfs_root *root,
+                     struct btrfs_key *node_keys,
+                     u64 *nodes, int lowest_level)
+{
+        struct extent_buffer *eb;
+        struct extent_buffer *parent;
+        struct btrfs_key key;
+        u64 bytenr;
+        u64 generation;
+        u32 blocksize;
+        int level;
+        int slot;
+        int key_match;
+        int ret;
+
+        eb = btrfs_lock_root_node(root);
+        ret = btrfs_cow_block(trans, root, eb, NULL, 0, &eb, 0);
+        BUG_ON(ret);
+
+        parent = eb;
+        while (1) {
+                level = btrfs_header_level(parent);
+                if (level == 0 || level <= lowest_level)
+                        break;
+
+                ret = bin_search(parent, &node_keys[lowest_level], level,
+                                 &slot);
+                if (ret && slot > 0)
+                        slot--;
+
+                bytenr = btrfs_node_blockptr(parent, slot);
+                if (nodes[level - 1] == bytenr)
+                        break;
+
+                blocksize = btrfs_level_size(root, level - 1);
+                generation = btrfs_node_ptr_generation(parent, slot);
+                btrfs_node_key_to_cpu(eb, &key, slot);
+                key_match = !memcmp(&key, &node_keys[level - 1], sizeof(key));
+
+                /*
+                 * if node keys match and node pointer hasn't been modified
+                 * in the running transaction, we can merge the path. for
+                 * reloc trees, the node pointer check is skipped, this is
+                 * because the reloc trees are fully controlled by the space
+                 * balance code, no one else can modify them.
+                 */
+                if (!nodes[level - 1] || !key_match ||
+                    (generation == trans->transid &&
+                     root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID)) {
+next_level:
+                        if (level == 1 || level == lowest_level + 1)
+                                break;
+
+                        eb = read_tree_block(root, bytenr, blocksize,
+                                             generation);
+                        btrfs_tree_lock(eb);
+
+                        ret = btrfs_cow_block(trans, root, eb, parent, slot,
+                                              &eb, 0);
+                        BUG_ON(ret);
+
+                        btrfs_tree_unlock(parent);
+                        free_extent_buffer(parent);
+                        parent = eb;
+                        continue;
+                }
+
+                if (generation == trans->transid) {
+                        u32 refs;
+                        BUG_ON(btrfs_header_owner(eb) !=
+                               BTRFS_TREE_RELOC_OBJECTID);
+                        /*
+                         * lock the block to keep __btrfs_cow_block from
+                         * changing the reference count.
+                         */
+                        eb = read_tree_block(root, bytenr, blocksize,
+                                             generation);
+                        btrfs_tree_lock(eb);
+
+                        ret = btrfs_lookup_extent_ref(trans, root, bytenr,
+                                                      blocksize, &refs);
+                        BUG_ON(ret);
+                        /*
+                         * if replace block whose reference count is one,
+                         * we have to "drop the subtree". so skip it for
+                         * simplicity
+                         */
+                        if (refs == 1) {
+                                btrfs_tree_unlock(eb);
+                                free_extent_buffer(eb);
+                                goto next_level;
+                        }
+                }
+
+                btrfs_set_node_blockptr(parent, slot, nodes[level - 1]);
+                btrfs_set_node_ptr_generation(parent, slot, trans->transid);
+                btrfs_mark_buffer_dirty(parent);
+
+                ret = btrfs_inc_extent_ref(trans, root,
+                                        nodes[level - 1],
+                                        blocksize, parent->start,
+                                        btrfs_header_owner(parent),
+                                        btrfs_header_generation(parent),
+                                        level - 1, 0);
+                BUG_ON(ret);
+                ret = btrfs_free_extent(trans, root, bytenr,
+                                        blocksize, parent->start,
+                                        btrfs_header_owner(parent),
+                                        btrfs_header_generation(parent),
+                                        level - 1, 0, 1);
+                BUG_ON(ret);
+
+                if (generation == trans->transid) {
+                        btrfs_tree_unlock(eb);
+                        free_extent_buffer(eb);
+                }
+                break;
+        }
+        btrfs_tree_unlock(parent);
+        free_extent_buffer(parent);
+        return 0;
+}
+
 /*
  * adjust the pointers going up the tree, starting at level
  * making sure the right key of each node is points to 'key'.