[dm-devel] Possible bug when releasing metadata snapshot in dm-thin

[Joe Thornber]

On Wed, Nov 19, 2014 at 05:21:52PM +0800, Teng-Feng Yang wrote:
> Hi all,
> 
> I accidentally run into this weird situation which looks like a bug to me.
> This bug can be reproduced every time with the following steps.
> 
> 1) Create a thin pool and a thin volume.
> 2) Write some data to this thin volume.
> 3) Reserve metadata snapshot by sending "reserve_metadata_snap" to pool.
> 4) Create a snapshot for the thin volume.
> 5) Release metadata snapshot by sending "release_metadata_snap" to pool
> 6) Remove both the snapshot and thin volume.
> 
> After these steps, pool blocks allocated to the thin volume are never
> returned to the pool. I trace the code of releasing metadata snapshot,
> and I might find the root cause of this. When reserving metadata
> snapshot, we will increase the reference count of data mapping root by
> 1. However, the subsequent changes to the data mapping tree will split
> the data mapping tree which results in increasing reference counts of
> all bottom level roots. When releasing metadata snapshot, we simply
> decrease the reference count of the old data mapping root without
> propagating these reference count decrements all the way down. IMHO,
> maybe we should call dm_btree_del() on the old data mapping root
> instead of dm_sm_dec_refcount().

Yep, that sounds likely.  I'll confirm and post a patch later.

Thanks,

- Joe