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

[Goldwyn Rodrigues]

Hi Srini,

Thanks for the reply.

On 01/08/2014 11:30 PM, Srinivas Eeda wrote:
>>>> >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.

Okay, but  why should node B do the cleanup/wipe when node A initiated 
the unlink()? Shouldn't it be done by node A? All node B should do is to 
write the inode and clear it from the cache. The sequence is 
synchronized by dentry_lock. Right?

We are performing ocfs2_inode_lock() anyways which is re-reading the 
inode from disk (for node A)


>
>>
>>>
>>>> >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

I think if we don't (ab)use OCFS2_INODE_MAYBE_ORPHANED, we should be 
better in this case as well, though I am not sure as of now.

Should I write a trial patch to explain better?

-- 
Goldwyn