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, 41 insertions, 4 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 5ace2c33f1a..9455974dabe 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;
 }