Re: [PATCH v4] btrfs: qgroup: Remove qgroup items along with subvolume deletion

[Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>]

On 2018/08/09 15:14, Qu Wenruo wrote:
> 
> 
> On 8/9/18 2:05 PM, Misono Tomohiro wrote:
>> On 2018/08/09 14:47, Qu Wenruo wrote:
>>>
>>>
>>> On 8/9/18 12:12 PM, Misono Tomohiro wrote:
>>>> When qgroup is on, subvolume deletion does not remove qgroup items
>>>> of the subvolume (qgroup info, limit, relation) from quota tree and
>>>> they need to get removed manually by "btrfs qgroup destroy".
>>>>
>>>> Since level 0 qgroup cannot be used/inherited by any other subvolume,
>>>> let's remove them automatically when subvolume is deleted
>>>> (to be precise, when the subvolume root is dropped).
>>>>
>>>> Note that qgroup becomes inconsistent in following case:
>>>>   1. qgroup relation exists
>>>>   2. and subvolume's excl != rref
>>>
>>> That's a little strange.
>>>
>>> If a subvolume is completely dropped, its excl should be the same rfer,
>>> all 0, and removing its relationship should not mark qgroup inconsistent.
>>>
>>> So the problem is the timing when btrfs_remove_qgroup() is called.
>>>
>>> Since qgroup accounting is only called at transaction commit time, and
>>> we're holding a trans handler, it's almost ensured we can't commit this
>>> transaction, thus the number is not updated yet (still not 0)
>>>
>>> So that's why qgroup is inconsistent.
>>>
>>> What about commit current transaction and then call btrfs_remove_qgroup()?
>>>
>>> (Sorry I didn't catch this problem last time I reviewed this patch)
>>
>> well, I'm little confusing about flow of transaction commit.
>> btrfs_drop_snapshot() is called from cleaner_kthread and
>> is it ok to commit transaction in it?
> 
> Not completely clear of the cleaner_kthread(), but from what I see in
> btrfs_drop_snapshot(), btrfs_end_transaction_throttle() itself could
> commit current transaction.
> 
> So in theory we should be OK to finish all the original work of
> btrfs_drop_snapshot(), and then commit current transaction, and finally
> do the qgroup cleanup work.
> 
> But I could totally be wrong, and feel free to point what I'm missing.

Thank you very much for explanation.
I changed code to commit transaction and it works,
so I hope next version will solve all the problem.

Thanks,
Misono

> 
> Thanks,
> Qu
> 
>>
>>>
>>> Thanks,
>>> Qu
>>>
>>>> In this case manual qgroup rescan is needed.
>>>>
>>>> Reviewed-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
>>>> Reviewed-by: Qu Wenruo <wqu@suse.com>
>>>> Signed-off-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
>>>> ---
>>>> Hi David,
>>>> It turned out that this patch may cause qgroup inconsistency in case
>>>> described above and need manual rescan. Since current code will keep 
>>>> qgroup items but not break qgroup consistency when deleting subvolume,
>>>> I cannot clearly say which behavior is better for qgroup usability.
>>>> Can I ask your opinion?
>>>>
>>>> v3 -> v4:
>>>>   Check return value of btrfs_remove_qgroup() and if it is 1,
>>>>   print message in syslog that fs needs qgroup rescan
>>>>
>>>>  fs/btrfs/extent-tree.c | 22 ++++++++++++++++++----
>>>>  1 file changed, 18 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>>>> index 9e7b237b9547..828d9e68047d 100644
>>>> --- a/fs/btrfs/extent-tree.c
>>>> +++ b/fs/btrfs/extent-tree.c
>>>> @@ -8871,12 +8871,13 @@ int btrfs_drop_snapshot(struct btrfs_root *root,
>>>>  	struct btrfs_root_item *root_item = &root->root_item;
>>>>  	struct walk_control *wc;
>>>>  	struct btrfs_key key;
>>>> +	u64 objectid = root->root_key.objectid;
>>>>  	int err = 0;
>>>>  	int ret;
>>>>  	int level;
>>>>  	bool root_dropped = false;
>>>>  
>>>> -	btrfs_debug(fs_info, "Drop subvolume %llu", root->objectid);
>>>> +	btrfs_debug(fs_info, "Drop subvolume %llu", objectid);
>>>>  
>>>>  	path = btrfs_alloc_path();
>>>>  	if (!path) {
>>>> @@ -9030,7 +9031,7 @@ int btrfs_drop_snapshot(struct btrfs_root *root,
>>>>  		goto out_end_trans;
>>>>  	}
>>>>  
>>>> -	if (root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID) {
>>>> +	if (objectid != BTRFS_TREE_RELOC_OBJECTID) {
>>>>  		ret = btrfs_find_root(tree_root, &root->root_key, path,
>>>>  				      NULL, NULL);
>>>>  		if (ret < 0) {
>>>> @@ -9043,8 +9044,7 @@ int btrfs_drop_snapshot(struct btrfs_root *root,
>>>>  			 *
>>>>  			 * The most common failure here is just -ENOENT.
>>>>  			 */
>>>> -			btrfs_del_orphan_item(trans, tree_root,
>>>> -					      root->root_key.objectid);
>>>> +			btrfs_del_orphan_item(trans, tree_root, objectid);
>>>>  		}
>>>>  	}
>>>>  
>>>> @@ -9056,6 +9056,20 @@ int btrfs_drop_snapshot(struct btrfs_root *root,
>>>>  		btrfs_put_fs_root(root);
>>>>  	}
>>>>  	root_dropped = true;
>>>> +
>>>> +	 /* Remove level-0 qgroup items since no other subvolume can use them */
>>>> +	ret = btrfs_remove_qgroup(trans, objectid);
>>>> +	if (ret == 1) {
>>>> +		/* This means qgroup becomes inconsistent by removing items */
>>>> +		btrfs_info(fs_info,
>>>> +		    "qgroup inconsistency found, need qgroup rescan");
>>>> +	} else if (ret == -EINVAL || ret == -ENOENT) {
>>>> +		/* qgroup is not enabled or already removed, just ignore this */
>>>> +	} else if (ret) {
>>>> +		btrfs_abort_transaction(trans, ret);
>>>> +		err = ret;
>>>> +	}
>>>> +
>>>>  out_end_trans:
>>>>  	btrfs_end_transaction_throttle(trans);
>>>>  out_free:
>>>>
>>>
>>
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