[Ocfs2-devel] What's the need of OCFS2_INODE_MAYBE_ORPHANED?

[Srinivas Eeda]

On 01/08/2014 07:12 PM, Goldwyn Rodrigues wrote:
> Hi Srini,
>
> On 01/08/2014 07:29 PM, Srinivas Eeda wrote:
>> Hi Goldwyn,
>>
>> On 01/08/2014 04:12 PM, Goldwyn Rodrigues wrote:
>>> Hi,
>>>
>>> >From the comments in fs/ocfs2/inode.h:90 it seems, this was used in
>>> legacy ocfs2 systems when a node received unlink votes. Since unlink
>>> votes has been done away with and replaced with open locks, is this
>>> flag still required? If yes, why?
>> My understanding is that unlink voting protocol was heavy. So the
>> following was done to address it.
>>
>> To do an unlink, dentry has to be removed. In order to do that the node
>> has to get EX lock on the dentry which means all other nodes have to
>> downconvert. In general EX lock on dentry is acquired only in unlink and
>> I assume rename case. So all nodes which down convert the lock mark
>> their inode OCFS2_INODE_MAYBE_ORPHANED. The only problem with this is
>> that dentry on a node can get purged because of memory pressure which
>> marks inode as OCFS2_INODE_MAYBE_ORPHANED even when no unlink was done
>> on this inode.
>>
>
> I think you are getting confused between dentry_lock (dentry_lockres) 
> and open lock (ip_open_lockres). AFAICS, dentry locks are used to 
> control the remote dentries.
I was trying to answer why we need OCFS2_INODE_MAYBE_ORPHANED flag, I 
guess I wasn't clear. I'll make an other attempt :).

One way for node A to tell node B that an unlink had happened on node A 
is by sending an explicit message(something similar to what we had in 
old release). When node B received such communication it marked inode 
with OCFS2_INODE_MAYBE_ORPHANED flag if it still had the inode in use.

The other way(current implementation) is to indirectly tell it by asking 
node B to purge dentry lockres. Once node B has been informed that 
dentry lock has to be released, it assumes inode might have been 
unlinked somewhere and marks the inode with OCFS2_INODE_MAYBE_ORPHANED flag.

So, we need OCFS2_INODE_MAYBE_ORPHANED flag to tell node B that it 
should finish the second phase of unlink(remove the inode from file 
system) when it closes the file.

>
>>
>>> >From my ongoing investigation of unlink() times, it seems this flag is
>>> causing the delay with releasing the open locks while downconverting
>>> dentry locks. The flag is set  _everytime_ a dentry downconvert is
>>> performed even if the file  is not scheduled to be deleted. If not, we
>>> can be smartly evict the inodes which are *not* to be deleted
>>> (i_nlink>0) by not offloading to ocfs2_wq. This way open lock will
>>> release faster speeding up unlink on the deleting node.
>>>
>>>
>> Are you referring to the delay caused by ocfs2_drop_dentry_lock queueing
>> dentry locks to dentry_lock_list ?. If that's the case, have you tried
>> removing following patches which introduced that behavior ? I think that
>> quota's deadlock bug might have to be addressed differently ?
>>
>> ea455f8ab68338ba69f5d3362b342c115bea8e13
>
> Yes, that should make some difference. Let me try that. However, I was 
> suggesting we do not set the OCFS2_INODE_MAYBE_ORPHANED flag in 
> ocfs2_dentry_convert_worker as well, but I am not sure of the 
> consequences and that is the reason I asked why it is used.
>
>> eb90e46458b08bc7c1c96420ca0eb4263dc1d6e5
>> bb44bf820481e19381ec549118e4ee0b89d56191
>
> I did not find these gits. Which tree are you referring to?

Sorry, my bad. Those commit id's were from my local repo. I meant
f7b1aa69be138ad9d7d3f31fa56f4c9407f56b6a and
5fd131893793567c361ae64cbeb28a2a753bbe35
>
>>
>> The above patches were leaving orphan files around which was causing a
>> big problem to some applications that removes lot of files which inturn
>> caused intermittent hangs
>>
>
>