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

[Josef Bacik <jbacik@fusionio.com>]

On Wed, Aug 08, 2012 at 09:10:17PM -0600, Miao Xie wrote:
> If we create several snapshots at the same time, the following BUG_ON() will be
> triggered.
> 
> 	kernel BUG at fs/btrfs/extent-tree.c:6047!
> 
> Steps to reproduce:
>  # 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
> 
> The reason is:
> Before transaction commit, some operations changed the fs tree and new tree
> blocks were allocated because of COW. We used the implicit non-shared back
> reference for those newly allocated tree blocks because they were not shared by
> two or more trees.
> 
> And then we created the first snapshot for the fs tree, according to the back
> reference rules, we also used implicit back refs for the child tree blocks of
> the root node of the fs tree, now those child nodes/leaves were shared by two
> trees.
> 
> Then We didn't deal with the delayed references, and continued to change the fs
> tree(created the second snapshot and inserted the dir item of the new snapshot
> into the fs tree). According to the rules of the back reference, we added full
> back refs for those tree blocks whose parents have be shared by two trees.
> Now some newly allocated tree blocks had two types of the references.
> 
> As we know, the delayed reference system handles these delayed references from
> back to front, and the full delayed reference is inserted after the implicit
> ones. So when we dealt with the back references of those newly allocated tree
> blocks, the full references was dealt with at first. And if the first reference
> is a shared back reference and the tree block that the reference points to is
> newly allocated, It would be considered as a tree block which is shared by two
> or more trees when it is allocated and should be a full back reference not a
> implicit one, the flag of its reference also should be set to FULL_BACKREF.
> But in fact, it was a non-shared tree block with a implicit reference at
> beginning, so it was not compulsory to set the flags to FULL_BACKREF. So BUG_ON
> was triggered.
> 
> We have several methods to fix this bug:
> 1. deal with delayed references after the snapshot is created and before we
>    change the source tree of the snapshot. This is the easiest and safest way.
> 2. modify the sort method of the delayed reference tree, make the full delayed
>    references be inserted before the implicit ones. It is also very easy, but
>    I don't know if it will introduce some problems or not.

Thanks for tracking this down, FWIW I like option 2 the most, it would be
intereseting to see if it does actually introduce new issues.  Thanks,

Josef