[dm-devel] dm thin: superblock may write succeed before other metadata blocks because of wirting metadata in async mode.

[Joe Thornber]

On Tue, Jun 19, 2018 at 09:11:06AM -0400, Mike Snitzer wrote:
> On Mon, May 21 2018 at  8:53pm -0400,
> Monty Pavel <monty_pavel at sina.com> wrote:
> 
> > 
> > If dm_bufio_write_dirty_buffers func is called by __commit_transaction
> > func and power loss happens during executing it, coincidencely
> > superblock wrote correctly but some metadata blocks didn't. The reason
> > is we write all metadata in async mode. We can guarantee that we send
> > superblock after other blocks but we cannot guarantee that superblock
> > write completely early than other blocks.
> > So, We need to commit other metadata blocks before change superblock.
> > 
> > Signed-off-by: Monty Pavel <monty_pavel at sina.com>
> > ---
> >  drivers/md/dm-thin-metadata.c |    8 ++++++++
> >  1 files changed, 8 insertions(+), 0 deletions(-)
> > 
> > diff --git a/drivers/md/dm-thin-metadata.c b/drivers/md/dm-thin-metadata.c
> > index 36ef284..897d7d6 100644
> > --- a/drivers/md/dm-thin-metadata.c
> > +++ b/drivers/md/dm-thin-metadata.c
> > @@ -813,6 +813,14 @@ static int __commit_transaction(struct dm_pool_metadata *pmd)
> >  	if (r)
> >  		return r;
> >  
> > +	r = dm_tm_commit(pmd->tm, sblock);
> > +	if (r)
> > +		return r;
> > +
> > +	r = superblock_lock(pmd, &sblock);
> > +	if (r)
> > +		return r;
> > +
> >  	disk_super = dm_block_data(sblock);
> >  	disk_super->time = cpu_to_le32(pmd->time);
> >  	disk_super->data_mapping_root = cpu_to_le64(pmd->root);

I don't believe you've tested this; sblock is passed to dm_tm_commit()
uninitialised, and you didn't even bother to remove the later (and correct)
call to dm_tm_commit().  See dm-transaction-manager.h for an explanation of
how the 2-phase commit works.

What is the issue that started you looking in this area?

- Joe