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>

1 files changed, 10 insertions, 0 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;