[PATCH 2/2] Btrfs: fix the snapshot that should not exist

[PATCH 2/2] Btrfs: fix the snapshot that should not exist

[Miao Xie]

The snapshot should be the image of the fs tree before it was created,
so the metadata of the snapshot should not exist in the its tree. But now, we
found the directory item and directory name index is in both the snapshot tree
and the fs tree. It introduces some problem and makes the users feel strange:

 # mkfs.btrfs /dev/sda1
 # mount /dev/sda1 /mnt
 # mkdir /mnt/1
 # cd /mnt/1
 # btrfs subvolume snapshot /mnt snap0
 # ll /mnt/1
 total 0
 drwxr-xr-x 1 root root 10 Ju1 24 12:11 1
			^^^
 # ll /mnt/1/snap0/
 total 0
 drwxr-xr-x 1 root root 10 Ju1 24 12:11 1
                        ^^^
                        It is also 10, but...
 # ll /mnt/1/snap0/1
 total 0
 [None]
 # cd /mnt/1/snap0/1/snap0
 [Enter a unexisted directory successfully...]

There is nothing in the directory 1 in snap0, but btrfs told the length of
this directory is 10. Beside that, we can enter an unexisted directory, it is
very strange to the users.

 # btrfs subvolume snapshot /mnt/1/snap0 /mnt/snap1
 # ll /mnt/1/snap0/1/
 total 0
 [None]
 # ll /mnt/snap1/1/
 total 0
 drwxr-xr-x 1 root root 0 Ju1 24 12:14 snap0

And the source of snap1 did have any directory in Directory 1, but snap1 have
a snap0, it is different between the source and the snapshot.

So I think we should insert directory item and directory name index and update
the parent inode as the last step of snapshot creation, and do not leave the
useless metadata in the tree.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
---
 fs/btrfs/transaction.c |   52 ++++++++++++++++++++++++++++++++++-------------
 1 files changed, 37 insertions(+), 15 deletions(-)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 6d89603..e9eceee 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -922,6 +922,8 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 	struct btrfs_root *parent_root;
 	struct btrfs_block_rsv *rsv;
 	struct inode *parent_inode;
+	struct btrfs_path *path;
+	struct btrfs_dir_item *dir_item;
 	struct dentry *parent;
 	struct dentry *dentry;
 	struct extent_buffer *tmp;
@@ -932,6 +934,12 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 	u64 objectid;
 	u64 root_flags;
 
+	path = btrfs_alloc_path();
+	if (!path) {
+		ret = pending->error = -ENOMEM;
+		goto path_alloc_fail;
+	}
+
 	new_root_item = kmalloc(sizeof(*new_root_item), GFP_NOFS);
 	if (!new_root_item) {
 		ret = pending->error = -ENOMEM;
@@ -973,22 +981,20 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 	 */
 	ret = btrfs_set_inode_index(parent_inode, &index);
 	BUG_ON(ret); /* -ENOMEM */
-	ret = btrfs_insert_dir_item(trans, parent_root,
-				dentry->d_name.name, dentry->d_name.len,
-				parent_inode, &key,
-				BTRFS_FT_DIR, index);
-	if (ret == -EEXIST) {
+
+	/* check if there is a file/dir which has the same name. */
+	dir_item = btrfs_lookup_dir_item(NULL, parent_root, path,
+					 btrfs_ino(parent_inode),
+					 dentry->d_name.name,
+					 dentry->d_name.len, 0);
+	if (dir_item != NULL && !IS_ERR(dir_item)) {
 		pending->error = -EEXIST;
 		goto fail;
-	} else if (ret) {
+	} else if (IS_ERR(dir_item)) {
+		ret = PTR_ERR(dir_item);
 		goto abort_trans;
 	}
-
-	btrfs_i_size_write(parent_inode, parent_inode->i_size +
-					 dentry->d_name.len * 2);
-	ret = btrfs_update_inode(trans, parent_root, parent_inode);
-	if (ret)
-		goto abort_trans;
+	btrfs_release_path(path);
 
 	/*
 	 * pull in the delayed directory update
@@ -1062,17 +1068,33 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 	ret = btrfs_reloc_post_snapshot(trans, pending);
 	if (ret)
 		goto abort_trans;
+
+	ret = btrfs_insert_dir_item(trans, parent_root,
+				    dentry->d_name.name, dentry->d_name.len,
+				    parent_inode, &key,
+				    BTRFS_FT_DIR, index);
+	/* We have check the name at the beginning, so it is impossible. */
+	BUG_ON(ret == -EEXIST);
+	if (ret)
+		goto abort_trans;
+
+	btrfs_i_size_write(parent_inode, parent_inode->i_size +
+					 dentry->d_name.len * 2);
+	ret = btrfs_update_inode(trans, parent_root, parent_inode);
+	if (ret)
+		goto abort_trans;
 fail:
 	dput(parent);
 	trans->block_rsv = rsv;
 no_free_objectid:
 	kfree(new_root_item);
 root_item_alloc_fail:
+	btrfs_free_path(path);
+path_alloc_fail:
 	btrfs_block_rsv_release(root, &pending->block_rsv, (u64)-1);
 	return ret;
 
 abort_trans:
-	dput(parent);
 	btrfs_abort_transaction(trans, root, ret);
 	goto fail;
 }
@@ -1386,13 +1408,13 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
 	 */
 	mutex_lock(&root->fs_info->reloc_mutex);
 
-	ret = btrfs_run_delayed_items(trans, root);
+	ret = create_pending_snapshots(trans, root->fs_info);
 	if (ret) {
 		mutex_unlock(&root->fs_info->reloc_mutex);
 		goto cleanup_transaction;
 	}
 
-	ret = create_pending_snapshots(trans, root->fs_info);
+	ret = btrfs_run_delayed_items(trans, root);
 	if (ret) {
 		mutex_unlock(&root->fs_info->reloc_mutex);
 		goto cleanup_transaction;
-- 
1.7.6.5

Re: [PATCH 2/2] Btrfs: fix the snapshot that should not exist

[Hidetoshi Seto]

(2012/07/26 15:57), Miao Xie wrote:
> The snapshot should be the image of the fs tree before it was created,
> so the metadata of the snapshot should not exist in the its tree. But now, we
> found the directory item and directory name index is in both the snapshot tree
> and the fs tree. It introduces some problem and makes the users feel strange:
> 
>  # mkfs.btrfs /dev/sda1
>  # mount /dev/sda1 /mnt
>  # mkdir /mnt/1
>  # cd /mnt/1
>  # btrfs subvolume snapshot /mnt snap0
>  # ll /mnt/1
>  total 0
>  drwxr-xr-x 1 root root 10 Ju1 24 12:11 1
> 			^^^
>  # ll /mnt/1/snap0/
>  total 0
>  drwxr-xr-x 1 root root 10 Ju1 24 12:11 1
>                         ^^^
>                         It is also 10, but...
>  # ll /mnt/1/snap0/1
>  total 0
>  [None]
>  # cd /mnt/1/snap0/1/snap0
>  [Enter a unexisted directory successfully...]

I confirmed that "mkdir snap0" failed with "File exists" and
that rmdir can remove the directory snap0. So it is a kind of
"invisible" rather than "unexisted".

> 
> There is nothing in the directory 1 in snap0, but btrfs told the length of
> this directory is 10. Beside that, we can enter an unexisted directory, it is
> very strange to the users.
> 
>  # btrfs subvolume snapshot /mnt/1/snap0 /mnt/snap1
>  # ll /mnt/1/snap0/1/
>  total 0
>  [None]
>  # ll /mnt/snap1/1/
>  total 0
>  drwxr-xr-x 1 root root 0 Ju1 24 12:14 snap0
> 
> And the source of snap1 did have any directory in Directory 1, but snap1 have
> a snap0, it is different between the source and the snapshot.
> 
> So I think we should insert directory item and directory name index and update
> the parent inode as the last step of snapshot creation, and do not leave the
> useless metadata in the tree.
> 
> Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
> ---

(snip)

> @@ -1062,17 +1068,33 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
>  	ret = btrfs_reloc_post_snapshot(trans, pending);
>  	if (ret)
>  		goto abort_trans;
> +
> +	ret = btrfs_insert_dir_item(trans, parent_root,
> +				    dentry->d_name.name, dentry->d_name.len,
> +				    parent_inode, &key,
> +				    BTRFS_FT_DIR, index);
> +	/* We have check the name at the beginning, so it is impossible. */
> +	BUG_ON(ret == -EEXIST);
> +	if (ret)
> +		goto abort_trans;
> +
> +	btrfs_i_size_write(parent_inode, parent_inode->i_size +
> +					 dentry->d_name.len * 2);
> +	ret = btrfs_update_inode(trans, parent_root, parent_inode);
> +	if (ret)
> +		goto abort_trans;
>  fail:
>  	dput(parent);
>  	trans->block_rsv = rsv;
>  no_free_objectid:
>  	kfree(new_root_item);
>  root_item_alloc_fail:
> +	btrfs_free_path(path);
> +path_alloc_fail:
>  	btrfs_block_rsv_release(root, &pending->block_rsv, (u64)-1);
>  	return ret;
>  
>  abort_trans:
> -	dput(parent);

I think you can remove this line in your 1/2 patch.

>  	btrfs_abort_transaction(trans, root, ret);
>  	goto fail;
>  }
> @@ -1386,13 +1408,13 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
>  	 */
>  	mutex_lock(&root->fs_info->reloc_mutex);
>  
> -	ret = btrfs_run_delayed_items(trans, root);
> +	ret = create_pending_snapshots(trans, root->fs_info);
>  	if (ret) {
>  		mutex_unlock(&root->fs_info->reloc_mutex);
>  		goto cleanup_transaction;
>  	}
>  
> -	ret = create_pending_snapshots(trans, root->fs_info);
> +	ret = btrfs_run_delayed_items(trans, root);
>  	if (ret) {
>  		mutex_unlock(&root->fs_info->reloc_mutex);
>  		goto cleanup_transaction;

It would be nice to have a patch description to tell why you
have to change the order here.

Thanks,
H.Seto

Re: [PATCH 2/2] Btrfs: fix the snapshot that should not exist

[David Sterba]

On Fri, Jul 27, 2012 at 04:52:21PM +0900, Hidetoshi Seto wrote:
> (2012/07/26 15:57), Miao Xie wrote:
> >  	btrfs_abort_transaction(trans, root, ret);
> >  	goto fail;
> >  }
> > @@ -1386,13 +1408,13 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
> >  	 */
> >  	mutex_lock(&root->fs_info->reloc_mutex);
> >  
> > -	ret = btrfs_run_delayed_items(trans, root);
> > +	ret = create_pending_snapshots(trans, root->fs_info);
> >  	if (ret) {
> >  		mutex_unlock(&root->fs_info->reloc_mutex);
> >  		goto cleanup_transaction;
> >  	}
> >  
> > -	ret = create_pending_snapshots(trans, root->fs_info);
> > +	ret = btrfs_run_delayed_items(trans, root);
> >  	if (ret) {
> >  		mutex_unlock(&root->fs_info->reloc_mutex);
> >  		goto cleanup_transaction;
> 
> It would be nice to have a patch description to tell why you
> have to change the order here.

Not only nice but necessary, as this order will cause corruption under
certain conditions. I'd like to hear the reason behind.


david

Re: [PATCH 2/2] Btrfs: fix the snapshot that should not exist

[Miao Xie]

On 	fri, 27 Jul 2012 14:29:57 +0200, David Sterba wrote:
> On Fri, Jul 27, 2012 at 04:52:21PM +0900, Hidetoshi Seto wrote:
>> (2012/07/26 15:57), Miao Xie wrote:
>>>  	btrfs_abort_transaction(trans, root, ret);
>>>  	goto fail;
>>>  }
>>> @@ -1386,13 +1408,13 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
>>>  	 */
>>>  	mutex_lock(&root->fs_info->reloc_mutex);
>>>  
>>> -	ret = btrfs_run_delayed_items(trans, root);
>>> +	ret = create_pending_snapshots(trans, root->fs_info);
>>>  	if (ret) {
>>>  		mutex_unlock(&root->fs_info->reloc_mutex);
>>>  		goto cleanup_transaction;
>>>  	}
>>>  
>>> -	ret = create_pending_snapshots(trans, root->fs_info);
>>> +	ret = btrfs_run_delayed_items(trans, root);
>>>  	if (ret) {
>>>  		mutex_unlock(&root->fs_info->reloc_mutex);
>>>  		goto cleanup_transaction;
>>
>> It would be nice to have a patch description to tell why you
>> have to change the order here.
> 
> Not only nice but necessary, as this order will cause corruption under
> certain conditions. I'd like to hear the reason behind.

What you worried is the corruption of the snapshots, right?
It is impossible because we will flush all the delayed items before the
creation of the snapshot(in create_pending_snapshot()). and we also will
force the tree to COW if we want to change it after it is snapshoted.
These two method will make sure the snapshots is healthy.

Thanks
Miao

Re: [PATCH 2/2] Btrfs: fix the snapshot that should not exist

[Miao Xie]

On fri, 27 Jul 2012 16:52:21 +0900, Hidetoshi Seto wrote:
>>  # ll /mnt/1/snap0/1
>>  total 0
>>  [None]
>>  # cd /mnt/1/snap0/1/snap0
>>  [Enter a unexisted directory successfully...]
> 
> I confirmed that "mkdir snap0" failed with "File exists" and
> that rmdir can remove the directory snap0. So it is a kind of
> "invisible" rather than "unexisted".

I think it is not like the typically invisible directories on linux, and in the users'
view, this directory should not exist, in other words, it is a unexisted directory to
the users, so I use "unexisted".

> (snip)
> 
>> @@ -1062,17 +1068,33 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
>>  	ret = btrfs_reloc_post_snapshot(trans, pending);
>>  	if (ret)
>>  		goto abort_trans;
>> +
>> +	ret = btrfs_insert_dir_item(trans, parent_root,
>> +				    dentry->d_name.name, dentry->d_name.len,
>> +				    parent_inode, &key,
>> +				    BTRFS_FT_DIR, index);
>> +	/* We have check the name at the beginning, so it is impossible. */
>> +	BUG_ON(ret == -EEXIST);
>> +	if (ret)
>> +		goto abort_trans;
>> +
>> +	btrfs_i_size_write(parent_inode, parent_inode->i_size +
>> +					 dentry->d_name.len * 2);
>> +	ret = btrfs_update_inode(trans, parent_root, parent_inode);
>> +	if (ret)
>> +		goto abort_trans;
>>  fail:
>>  	dput(parent);
>>  	trans->block_rsv = rsv;
>>  no_free_objectid:
>>  	kfree(new_root_item);
>>  root_item_alloc_fail:
>> +	btrfs_free_path(path);
>> +path_alloc_fail:
>>  	btrfs_block_rsv_release(root, &pending->block_rsv, (u64)-1);
>>  	return ret;
>>  
>>  abort_trans:
>> -	dput(parent);
> 
> I think you can remove this line in your 1/2 patch.

Yes. will be modified in the next version of the patches.

> 
>>  	btrfs_abort_transaction(trans, root, ret);
>>  	goto fail;
>>  }
>> @@ -1386,13 +1408,13 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
>>  	 */
>>  	mutex_lock(&root->fs_info->reloc_mutex);
>>  
>> -	ret = btrfs_run_delayed_items(trans, root);
>> +	ret = create_pending_snapshots(trans, root->fs_info);
>>  	if (ret) {
>>  		mutex_unlock(&root->fs_info->reloc_mutex);
>>  		goto cleanup_transaction;
>>  	}
>>  
>> -	ret = create_pending_snapshots(trans, root->fs_info);
>> +	ret = btrfs_run_delayed_items(trans, root);
>>  	if (ret) {
>>  		mutex_unlock(&root->fs_info->reloc_mutex);
>>  		goto cleanup_transaction;
> 
> It would be nice to have a patch description to tell why you
> have to change the order here.

OK, I will add comment in the next version of the patches.

Thanks for your review.

Miao