Btrfs: fix tree corruption after multi-thread snapshots and inode_cache flush

The btrfs snapshotting code requires that once a root has been
snapshotted, we don't change it during a commit.

But there are two cases to lead to tree corruptions:

1) multi-thread snapshots can commit serveral snapshots in a transaction,
   and this may change the src root when processing the following pending
   snapshots, which lead to the former snapshots corruptions;

2) the free inode cache was changing the roots when it root the cache,
   which lead to corruptions.

This fixes things by making sure we force COW the block after we create a
snapshot during commiting a transaction, then any changes to the roots
will result in COW, and we get all the fs roots and snapshot roots to be
consistent.

Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 0fe615e4ea387582acc06f60cac81366f23fe069..dede441bdeee2678225187bece170710abe1a9b4 100644 (file)
--- 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;
 }

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index b9ba59ff9292559330745b8afb41eefacfbf1615..b1cb3c0524844872f0e15e6f4a449e193c881b47 100644 (file)
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1271,6 +1271,8 @@ struct btrfs_root {
         * for stat.  It may be used for more later
         */
        dev_t anon_dev;
+
+       int force_cow;
 };
 
 struct btrfs_ioctl_defrag_range_args {

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 6a0574e923bc6a33b4965976328222dcf0a61cb0..81376d94cd3c6a4639ebef35df501dbefbfb2435 100644 (file)
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -785,6 +785,10 @@ static noinline int commit_fs_roots(struct btrfs_trans_handle *trans,
 
                        btrfs_save_ino_cache(root, trans);
 
+                       /* see comments in should_cow_block() */
+                       root->force_cow = 0;
+                       smp_wmb();
+
                        if (root->commit_root != root->node) {
                                mutex_lock(&root->fs_commit_mutex);
                                switch_commit_root(root);
@@ -947,6 +951,10 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
        btrfs_tree_unlock(old);
        free_extent_buffer(old);
 
+       /* see comments in should_cow_block() */
+       root->force_cow = 1;
+       smp_wmb();
+
        btrfs_set_root_node(new_root_item, tmp);
        /* record when the snapshot was created in key.offset */
        key.offset = trans->transid;