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Date:   Wed, 4 Aug 2021 19:00:20 -0500 (CDT)
From:   Timothy Pearson <tpearson@raptorengineering.com>
To:     linux-nfs <linux-nfs@vger.kernel.org>
Message-ID: <985631970.48634.1628121620017.JavaMail.zimbra@raptorengineeringinc.com>
Subject: Callback slot table overflowed
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Thread-Topic: Callback slot table overflowed
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All,

We've hit an odd issue after upgrading a main NFS server from Debian Stretch to Debian Buster.  In both cases the 5.13.4 kernel was used, however after the upgrade none of our ARM thin clients can mount their root filesystems -- early in the boot process I/O errors are returned immediately following "Callback slot table overflowed" in the client dmesg.

I am unable to find any useful information on this "Callback slot table overflowed" message, and have no idea why it is only impacting our ARM (armel) clients.  Both 4.14 and 5.3 on the client side show the issue, other client kernel versions were not tested.

Curiously, increasing the rsize/wsize values to 65536 or higher reduces (but does not eliminate) the number of callback overflow messages.

The server is a ppc64el 64k page host, and none of our pcc64el or amd64 thin clients are experiencing any problems.  Nothing of interest appears in the server message log.

Any troubleshooting hints would be most welcome.

Thank you!