Btrfs: Do snapshot deletion in smaller chunks.

Before, snapshot deletion was a single atomic unit.  This caused considerable
lock contention and required an unbounded amount of space.  Now,
the drop_progress field in the root item is used to indicate how far along
snapshot deletion is, and to resume where it left off.

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

4 files changed, 91 insertions, 31 deletions

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 19a30b7c3a28..aa824e2c521f 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -659,9 +659,16 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root
         struct buffer_head *b;
         struct buffer_head *cow_buf;
         struct btrfs_node *c;
+        struct btrfs_root_item *root_item = &root->root_item;
         int slot;
         int ret;
         int level;
+        u8 lowest_level = 0;
+
+        if (btrfs_root_refs(root_item) == 0 && root->ref_cows) {
+                lowest_level = root_item->drop_level;
+                WARN_ON(ins_len || cow);
+        }
 
         WARN_ON(p->nodes[0] != NULL);
         WARN_ON(!mutex_is_locked(&root->fs_info->fs_mutex));
@@ -718,6 +725,9 @@ again:
                                 slot = p->slots[level];
                                 BUG_ON(btrfs_header_nritems(&c->header) == 1);
                         }
+                        /* this is only true while dropping a snapshot */
+                        if (level == lowest_level)
+                                break;
                         b = read_tree_block(root, btrfs_node_blockptr(c, slot));
                 } else {
                         struct btrfs_leaf *l = (struct btrfs_leaf *)c;
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 0287bd51d87c..73c2e75a136d 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -333,10 +333,10 @@ struct btrfs_root {
         u64 objectid;
         u64 last_trans;
         u32 blocksize;
-        int ref_cows;
         u32 type;
         u64 highest_inode;
         u64 last_inode_alloc;
+        int ref_cows;
 };
 
 /* the lower bits in the key flags defines the item type */
@@ -1073,7 +1073,7 @@ int btrfs_insert_empty_item(struct btrfs_trans_handle *trans, struct btrfs_root
 int btrfs_next_leaf(struct btrfs_root *root, struct btrfs_path *path);
 int btrfs_leaf_free_space(struct btrfs_root *root, struct btrfs_leaf *leaf);
 int btrfs_drop_snapshot(struct btrfs_trans_handle *trans, struct btrfs_root
-                        *root, struct buffer_head *snap);
+                        *root);
 /* root-item.c */
 int btrfs_del_root(struct btrfs_trans_handle *trans, struct btrfs_root *root,
                    struct btrfs_key *key);
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 5ace2c33f1aa..9455974dabea 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -1561,12 +1561,21 @@ static int walk_up_tree(struct btrfs_trans_handle *trans, struct btrfs_root
         int i;
         int slot;
         int ret;
+        struct btrfs_root_item *root_item = &root->root_item;
+
         for(i = *level; i < BTRFS_MAX_LEVEL - 1 && path->nodes[i]; i++) {
                 slot = path->slots[i];
                 if (slot < btrfs_header_nritems(
                     btrfs_buffer_header(path->nodes[i])) - 1) {
+                        struct btrfs_node *node;
+                        node = btrfs_buffer_node(path->nodes[i]);
                         path->slots[i]++;
                         *level = i;
+                        WARN_ON(*level == 0);
+                        memcpy(&root_item->drop_progress,
+                               &node->ptrs[path->slots[i]].key,
+                               sizeof(root_item->drop_progress));
+                        root_item->drop_level = i;
                         return 0;
                 } else {
                         ret = btrfs_free_extent(trans, root,
@@ -1587,7 +1596,7 @@ static int walk_up_tree(struct btrfs_trans_handle *trans, struct btrfs_root
  * decremented.
  */
 int btrfs_drop_snapshot(struct btrfs_trans_handle *trans, struct btrfs_root
-                        *root, struct buffer_head *snap)
+                        *root)
 {
         int ret = 0;
         int wret;
@@ -1595,14 +1604,33 @@ int btrfs_drop_snapshot(struct btrfs_trans_handle *trans, struct btrfs_root
         struct btrfs_path *path;
         int i;
         int orig_level;
+        int num_walks = 0;
+        struct btrfs_root_item *root_item = &root->root_item;
 
         path = btrfs_alloc_path();
         BUG_ON(!path);
 
-        level = btrfs_header_level(btrfs_buffer_header(snap));
+        level = btrfs_header_level(btrfs_buffer_header(root->node));
         orig_level = level;
-        path->nodes[level] = snap;
-        path->slots[level] = 0;
+        if (btrfs_disk_key_objectid(&root_item->drop_progress) == 0) {
+                path->nodes[level] = root->node;
+                path->slots[level] = 0;
+        } else {
+                struct btrfs_key key;
+                struct btrfs_disk_key *found_key;
+                struct btrfs_node *node;
+                btrfs_disk_key_to_cpu(&key, &root_item->drop_progress);
+                wret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
+                if (ret < 0) {
+                        ret = wret;
+                        goto out;
+                }
+                level = root_item->drop_level;
+                node = btrfs_buffer_node(path->nodes[level]);
+                found_key = &node->ptrs[path->slots[level]].key;
+                WARN_ON(memcmp(found_key, &root_item->drop_progress,
+                               sizeof(*found_key)));
+        }
         while(1) {
                 wret = walk_down_tree(trans, root, path, &level);
                 if (wret > 0)
@@ -1615,12 +1643,21 @@ int btrfs_drop_snapshot(struct btrfs_trans_handle *trans, struct btrfs_root
                         break;
                 if (wret < 0)
                         ret = wret;
+                num_walks++;
+                if (num_walks > 10) {
+                        struct btrfs_key key;
+                        btrfs_disk_key_to_cpu(&key, &root_item->drop_progress);
+                        ret = -EAGAIN;
+                        get_bh(root->node);
+                        break;
+                }
         }
         for (i = 0; i <= orig_level; i++) {
                 if (path->nodes[i]) {
                         btrfs_block_release(root, path->nodes[i]);
                 }
         }
+out:
         btrfs_free_path(path);
         return ret;
 }
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 0f494fe365cb..498626470a04 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -213,10 +213,7 @@ static int wait_for_commit(struct btrfs_root *root,
 
 struct dirty_root {
         struct list_head list;
-        struct btrfs_key snap_key;
-        struct buffer_head *commit_root;
         struct btrfs_root *root;
-        int free_on_drop;
 };
 
 int btrfs_add_dead_root(struct btrfs_root *root, struct list_head *dead_list)
@@ -226,10 +223,7 @@ int btrfs_add_dead_root(struct btrfs_root *root, struct list_head *dead_list)
         dirty = kmalloc(sizeof(*dirty), GFP_NOFS);
         if (!dirty)
                 return -ENOMEM;
-        memcpy(&dirty->snap_key, &root->root_key, sizeof(root->root_key));
-        dirty->commit_root = root->node;
         dirty->root = root;
-        dirty->free_on_drop = 1;
         list_add(&dirty->list, dead_list);
         return 0;
 }
@@ -241,7 +235,6 @@ static int add_dirty_roots(struct btrfs_trans_handle *trans,
         struct dirty_root *dirty;
         struct btrfs_root *gang[8];
         struct btrfs_root *root;
-        struct btrfs_root_item tmp_item;
         int i;
         int ret;
         int err = 0;
@@ -267,13 +260,16 @@ static int add_dirty_roots(struct btrfs_trans_handle *trans,
                         }
                         dirty = kmalloc(sizeof(*dirty), GFP_NOFS);
                         BUG_ON(!dirty);
-                        memcpy(&dirty->snap_key, &root->root_key,
-                               sizeof(root->root_key));
-                        dirty->commit_root = root->commit_root;
+                        dirty->root = kmalloc(sizeof(*dirty->root), GFP_NOFS);
+                        BUG_ON(!dirty->root);
+
+                        memset(&root->root_item.drop_progress, 0,
+                               sizeof(struct btrfs_disk_key));
+                        root->root_item.drop_level = 0;
+
+                        memcpy(dirty->root, root, sizeof(*root));
+                        dirty->root->node = root->commit_root;
                         root->commit_root = NULL;
-                        dirty->root = root;
-                        dirty->free_on_drop = 0;
-                        memcpy(&tmp_item, &root->root_item, sizeof(tmp_item));
 
                         root->root_key.offset = root->fs_info->generation;
                         btrfs_set_root_blocknr(&root->root_item,
@@ -283,17 +279,21 @@ static int add_dirty_roots(struct btrfs_trans_handle *trans,
                                                 &root->root_item);
                         if (err)
                                 break;
-                        refs = btrfs_root_refs(&tmp_item);
-                        btrfs_set_root_refs(&tmp_item, refs - 1);
+
+                        refs = btrfs_root_refs(&dirty->root->root_item);
+                        btrfs_set_root_refs(&dirty->root->root_item, refs - 1);
                         err = btrfs_update_root(trans, root->fs_info->tree_root,
-                                                &dirty->snap_key,
-                                                &tmp_item);
+                                                &dirty->root->root_key,
+                                                &dirty->root->root_item);
 
                         BUG_ON(err);
-                        if (refs == 1)
+                        if (refs == 1) {
                                 list_add(&dirty->list, list);
-                        else
+                        } else {
+                                WARN_ON(1);
+                                kfree(dirty->root);
                                 kfree(dirty);
+                        }
                 }
         }
         return err;
@@ -305,23 +305,36 @@ static int drop_dirty_roots(struct btrfs_root *tree_root,
         struct dirty_root *dirty;
         struct btrfs_trans_handle *trans;
         int ret = 0;
+        int err;
+
         while(!list_empty(list)) {
                 mutex_lock(&tree_root->fs_info->fs_mutex);
                 dirty = list_entry(list->next, struct dirty_root, list);
                 list_del_init(&dirty->list);
 
-                trans = btrfs_start_transaction(tree_root, 1);
-                ret = btrfs_drop_snapshot(trans, dirty->root,
-                                          dirty->commit_root);
+                while(1) {
+                        trans = btrfs_start_transaction(tree_root, 1);
+                        ret = btrfs_drop_snapshot(trans, dirty->root);
+                        if (ret != -EAGAIN) {
+                                break;
+                        }
+                        err = btrfs_update_root(trans,
+                                        tree_root,
+                                        &dirty->root->root_key,
+                                        &dirty->root->root_item);
+                        if (err)
+                                ret = err;
+                        ret = btrfs_end_transaction(trans, tree_root);
+                        BUG_ON(ret);
+                }
                 BUG_ON(ret);
-                ret = btrfs_del_root(trans, tree_root, &dirty->snap_key);
+                ret = btrfs_del_root(trans, tree_root, &dirty->root->root_key);
                 if (ret)
                         break;
                 ret = btrfs_end_transaction(trans, tree_root);
                 BUG_ON(ret);
 
-                if (dirty->free_on_drop)
-                        kfree(dirty->root);
+                kfree(dirty->root);
                 kfree(dirty);
                 mutex_unlock(&tree_root->fs_info->fs_mutex);
                 btrfs_btree_balance_dirty(tree_root);