Btrfs: fix broken nocow after balance

Balance will create reloc_root for each fs root, and it's going to
record last_snapshot to filter shared blocks.  The side effect of
setting last_snapshot is to break nocow attributes of files.

Since the extents are not shared by the relocation tree after the balance,
we can recover the old last_snapshot safely if no one snapshoted the
source tree. We fix the above problem by this way.

Reported-by: Kyle Gates <kylegates@hotmail.com>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

1 files changed, 44 insertions, 0 deletions

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index aa559f1161df..4a404b44b0ad 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -1305,6 +1305,7 @@ static struct btrfs_root *create_reloc_root(struct btrfs_trans_handle *trans,
         struct extent_buffer *eb;
         struct btrfs_root_item *root_item;
         struct btrfs_key root_key;
+        u64 last_snap = 0;
         int ret;
 
         root_item = kmalloc(sizeof(*root_item), GFP_NOFS);
@@ -1320,6 +1321,7 @@ static struct btrfs_root *create_reloc_root(struct btrfs_trans_handle *trans,
                                       BTRFS_TREE_RELOC_OBJECTID);
                 BUG_ON(ret);
 
+                last_snap = btrfs_root_last_snapshot(&root->root_item);
                 btrfs_set_root_last_snapshot(&root->root_item,
                                              trans->transid - 1);
         } else {
@@ -1345,6 +1347,12 @@ static struct btrfs_root *create_reloc_root(struct btrfs_trans_handle *trans,
                 memset(&root_item->drop_progress, 0,
                        sizeof(struct btrfs_disk_key));
                 root_item->drop_level = 0;
+                /*
+                 * abuse rtransid, it is safe because it is impossible to
+                 * receive data into a relocation tree.
+                 */
+                btrfs_set_root_rtransid(root_item, last_snap);
+                btrfs_set_root_otransid(root_item, trans->transid);
         }
 
         btrfs_tree_unlock(eb);
@@ -2272,8 +2280,12 @@ void free_reloc_roots(struct list_head *list)
 static noinline_for_stack
 int merge_reloc_roots(struct reloc_control *rc)
 {
+        struct btrfs_trans_handle *trans;
         struct btrfs_root *root;
         struct btrfs_root *reloc_root;
+        u64 last_snap;
+        u64 otransid;
+        u64 objectid;
         LIST_HEAD(reloc_roots);
         int found = 0;
         int ret = 0;
@@ -2307,12 +2319,44 @@ again:
                 } else {
                         list_del_init(&reloc_root->root_list);
                 }
+
+                /*
+                 * we keep the old last snapshod transid in rtranid when we
+                 * created the relocation tree.
+                 */
+                last_snap = btrfs_root_rtransid(&reloc_root->root_item);
+                otransid = btrfs_root_otransid(&reloc_root->root_item);
+                objectid = reloc_root->root_key.offset;
+
                 ret = btrfs_drop_snapshot(reloc_root, rc->block_rsv, 0, 1);
                 if (ret < 0) {
                         if (list_empty(&reloc_root->root_list))
                                 list_add_tail(&reloc_root->root_list,
                                               &reloc_roots);
                         goto out;
+                } else if (!ret) {
+                        /*
+                         * recover the last snapshot tranid to avoid
+                         * the space balance break NOCOW.
+                         */
+                        root = read_fs_root(rc->extent_root->fs_info,
+                                            objectid);
+                        if (IS_ERR(root))
+                                continue;
+
+                        if (btrfs_root_refs(&root->root_item) == 0)
+                                continue;
+
+                        trans = btrfs_join_transaction(root);
+                        BUG_ON(IS_ERR(trans));
+
+                        /* Check if the fs/file tree was snapshoted or not. */
+                        if (btrfs_root_last_snapshot(&root->root_item) ==
+                            otransid - 1)
+                                btrfs_set_root_last_snapshot(&root->root_item,
+                                                             last_snap);
+                                
+                        btrfs_end_transaction(trans, root);
                 }
         }