[Ocfs2-devel] [RFC] ocfs2: Remove j_trans_barrier

[Tao Ma]

Hi Joel,

On 11/25/2010 06:08 PM, Joel Becker wrote:
> On Wed, Oct 20, 2010 at 02:08:17PM +0800, Tao Ma wrote:
>> 	j_trans_barrier in ocfs2 is used to protect some journal operations
>> in ocfs2. So normally, it is used as belows:
>> 1. In journal transaction. When we start a transaction, We will
>> down_read it and j_num_trans will be increased accordingly(in case
>> of a cluster environment). It will be up_read when we do
>> ocfs2_commit_trans.
>> 2. In ocfs2_commit_cache, we will down_write it and then call
>> jbd2_journal_flush, increase j_trans_id, reset j_num_trans and
>> finally call up_write. This function is used by thread ocfs2cmt.
>
> <snip>  slow filesystem...</snip>
>
>> My solution is that:
>> 1. remove j_trans_barrier
>> 2. Add a flag ci_checkpointing in ocfs2_caching_info:
>>     1) When we find this caching_info needs checkpoint, set this flag
>> and start the checkpointing(in ocfs2_ci_checkpointed). And the
>> downconvert request will be requeued so that we can check and clear
>> this flag next time it is handled.
>>     2) Clear the flag when there is no need for checkpointing this
>> ci(also in ocfs2_ci_checkpointed) during check_downconvert.
>> 3. make sure when we journal_access some blocks, the caching_info
>> can't be in the state of checkpointing. I think if we are
>> checkpointing an caching_info, we shouldn't be able to
>> journal_access it since it is just required to downconvert and we
>> shouldn't have the lock now? So perhaps a BUG_ON should work?
>
> Tao,
> 	I'm sorry I haven't responded sooner.  This proposal didn't
> strike me as quite right, and I didn't have time to think about it.
> I have a couple of concerns.
Never mind. I knew you had a lot of stuff to handle with these days.
> 	First, we don't always checkpoint from a downconvert.  We do it
> in clear_inode() as well, when we are flushing an inode from cache.
> This may not have anything to do with the lock we're caring about, eg on
> other inodes.  What I mean is, the caching info for the inode we care
> about may not be checkpointing, but the journal as a whole is.  We need
> to stop all action while that is happening.
Sorry I don't get your last sentense. Could you please describe it in 
detail? Yes, clear_inode does do checkpointing, but actually the whole 
thing is self-contained. In ocfs2_checkpoint_inode, it can checkpoint 
the inode by itself and has no relationship with downconvert.
> 	Second, there is the flip side.  How do we wait until all open
> transactions are complete before checkpointing?  The down_write() in
> ocfs2_commit_cache() blocks until all open transactions up_read().  In
> your scheme, there is no care taken for open transactions against the
> journal.  Remember, the journal is global to the node.
yes, I was thinking of that too. But finally I got that we don't need to 
care for it. As we have agreed above, there are 2 places we do 
checkpoint for an inode. As for clear_inode, we don't care since it is 
going to be cleared and no transaction could be opened against that. 
Another is downconvert, in which case we shouldn't be able to open a 
transaction and access that caching_info(we should always get the 
cluster lock before we do access it). We can add a BUG_ON to 
journal_access which can facilitate us to find the case that we don't 
have the lock while accessing it.

btw, I have some draft patch for it, I haven't tested it much these 
days. But if you are interested, I can send it to the mail list for more 
review.

Regards,
Tao