-----Ursprüngliche Nachricht-----
Von: Darrick J. Wong [mailto:darrick.wong@oracle.com] 
Gesendet: Freitag, 7. Juli 2017 05:47
An: Markus Stockhausen
Cc: 'linux-xfs@vger.kernel.org'
Betreff: Re: XFS hang - 4.4.73 longterm

quoted

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This is the known deadlock in SEEK_HOLE/SEEK_DATA; I have patches queued to fix it in 4.13, as soon as the dust settles and I send the pull req.

Short, precise, frightening.

Can you advise what will the best option to avoid that error. 
First things that come to my mind would be:

- get back to original 3.10 stable kernel from CentOS Distro
- lower NFS mount version
- Maybe remove some single patch that introduced the error?

Unfortunately, SEEK_HOLE (and the deadlocky code) go all the way back to the beginning of SEEK_HOLE support, so I don't know that 
there's a good workaround for this.  There's not really a way to shut off support for it, either, that doesn't involve gross code surgery.

Not good news. Finally I disabled SEEK support in nfsd by partially reverting 3.18 patch 
24bab491220faa446d94 "NFSD: Implement SEEK". While before NFS/XFS crashed with 
active VM snapshots and heavy disk writes after minutes it now runs for 2 days of 
intensive testing without errors.

At least now I have full discard support in side VMs (via hole punching of disk files).
I'm not familiar with SEEK_HOLE/SEEK_data feature. Do you see negative impact
if disabling these? In contrast to falling back to NFS 4 without seek + discard support.

Markus