On 09/29/2011 03:19 PM, Miao Xie wrote:
> On thu, 29 Sep 2011 14:46:20 +0800, Yan, Zheng wrote:
>> 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.
>
> Not orphan inode IDs.
> The inode IDs in the free_ino_pinned tree are also freed after the fs/file tree commit.
>
Any reason free_ino_pinned is 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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>>
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