On 09/29/2011 02:47 PM, Miao Xie wrote:
> On thu, 29 Sep 2011 12:25:56 +0800, Yan, Zheng wrote:
>> 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.
>
> I think this feature, that the inode cache is written out after creating snapshot,
> was implemented on purpose. Because some i-node IDs are freed after their tree is
> committed, and so the newly created snapshot must cache the i-node ID again to
> guarantee the inode cache is right, even though we write out the inode cache of
> the trees before they are snapshotted. So it is unnecessary to make the inode cache
> be written out before creating snapshot.
>
When opening the newly created snapshot, orphan cleanup will find these
freed-after-commited inodes and update the inode cache. So technically,
rescan is not required.
> Li, am I right?
>
> Thanks
> Miao
>
>>
>>> 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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