Re: [RFC PATCH] Btrfs: fix full backref problem when inserting shared block reference

[Miao Xie <miaox@cn.fujitsu.com>]

On Wed, 30 Jan 2013 20:23:22 +0200, Alex Lyakas wrote:
> Hi Miao,
> I was following this thread in the past, but I did not understand it
> fully, maybe you can explain?
> 
>>>>>  # mkfs.btrfs <partition>
>>>>>  # mount <partition> <mnt>
>>>>>  # cd <mnt>
>>>>>  # for ((i=0;i<2400;i++)); do touch long_name_to_make_tree_more_deep$i; done
>>>>>  # for ((i=0; i<4; i++))
>>>>>  > do
>>>>>  > mkdir $i
>>>>>  > for ((j=0; j<200; j++))
>>>>>  > do
>>>>>  > btrfs sub snap . $i/$j
>>>>>  > done &
>>>>>  > done
>>>>
>>>> snapshot creation has a critical section.  Once we copy a given root to
>>>> its snapshot, we're not allowed to change it until the transaction
>>>> is fully committed.
> 
> Is the limitation that if we are creating a snap B of root A, and
> placing the root of B somewhere into the tree of A, then we can do
> this only once per transaction? Does this limitation still exist or
> your fix fixes it?

The limitation is the snapshoted subvolume can not be changed until the transaction
is committed. That is we can not insert anything(including the root of B and the
directory item/index of B) into the tree of A after snap B is created.

This limitation was fixed.

> Also, according to your reproducer, each "btrfs sub snap" will
> start/join a transaction, but then it will call
> btrfs_commit_transaction() and not btrfs_commit_transaction_async(),
> so it will wait until the transaction commits. So how it may happen
> that you create more than one snap in the same transaction with your
> reproducer?

run several tasks, and each task create snapshots repeatedly in its own
directory.
(If we create snapshots in the same directory, the i_mutex of the directory
 will make the process serialized)

> The reason I am asking, is that I want to try to write code that
> creates several snaps in one transaction and only then commits. Should
> this be possible or there is some limitation, like I mentioned above?

As far as I know, it is possible, there is no limitation now.

Thanks
Miao