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-rw-r--r--fs/btrfs/ctree.c17
1 files changed, 16 insertions, 1 deletions
diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 0fe615e4ea38..dede441bdeee 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -514,10 +514,25 @@ static inline int should_cow_block(struct btrfs_trans_handle *trans,
struct btrfs_root *root,
struct extent_buffer *buf)
{
+ /* ensure we can see the force_cow */
+ smp_rmb();
+
+ /*
+ * We do not need to cow a block if
+ * 1) this block is not created or changed in this transaction;
+ * 2) this block does not belong to TREE_RELOC tree;
+ * 3) the root is not forced COW.
+ *
+ * What is forced COW:
+ * when we create snapshot during commiting the transaction,
+ * after we've finished coping src root, we must COW the shared
+ * block to ensure the metadata consistency.
+ */
if (btrfs_header_generation(buf) == trans->transid &&
!btrfs_header_flag(buf, BTRFS_HEADER_FLAG_WRITTEN) &&
!(root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID &&
- btrfs_header_flag(buf, BTRFS_HEADER_FLAG_RELOC)))
+ btrfs_header_flag(buf, BTRFS_HEADER_FLAG_RELOC)) &&
+ !root->force_cow)
return 0;
return 1;
}