On 09/29/2011 10:00 AM, Liu Bo wrote:
> 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.
>
For the case 2, the free inode cache of newly created snapshot is invalid.
So it's better to avoid modifying snapshotted trees.
> 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@xxxxxxxxxxxxxx>
> Signed-off-by: Miao Xie <miaox@xxxxxxxxxxxxxx>
> ---
> fs/btrfs/ctree.c | 17 ++++++++++++++++-
> fs/btrfs/ctree.h | 2 ++
> fs/btrfs/transaction.c | 8 ++++++++
> 3 files changed, 26 insertions(+), 1 deletions(-)
>
> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
> index 011cab3..49dad7d 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;
> }
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 03912c5..bece0df 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -1225,6 +1225,8 @@ 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 7dc36fa..bf6e2b3 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -816,6 +816,10 @@ 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);
> @@ -976,6 +980,10 @@ 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;
--
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