[SRU][Disco][PATCH 0/1] NFSv4.1: Interrupted connections cause high bandwidth RPC ping-pong between client and server

[Matthew Ruffell]

BugLink: https://bugs.launchpad.net/bugs/1828978

[Impact]

There is a bug in NFS v4.1 that causes a large amount of RPC calls between a 
client and server when a previous RPC call is interrupted. This uses a large 
amount of bandwidth and can saturate the network.

The symptoms are so:

* On NFS clients:
Attempts to access mounted NFS shares associated with the affected server block
indefinitely.
 
* On the network:
A storm of repeated RPCs between NFS client and server uses a lot of bandwidth.
Each RPC is acknoledged by the server with an NFS4ERR_SEQ_MISORDERED error.

* Other NFS clients connected to the same NFS server:
Performance drops dramatically.

This occurs during a "false retry", when a client attempts to make a new RPC 
call using a slot+sequence number that references an older, cached call. This 
happens when a user process interrupts an RPC call that is in progress.

[Fix]

This was fixed in 5.1 upstream with the below commit:

commit 3453d5708b33efe76f40eca1c0ed60923094b971
Author: Trond Myklebust <trond.myklebust at hammerspace.com>
Date:   Wed Jun 20 17:53:34 2018 -0400
Subject: NFSv4.1: Avoid false retries when RPC calls are interrupted

The fix is to pre-emptively increment the sequence number if an RPC call is 
interrupted, and to address corner cases we interpret the NFS4ERR_SEQ_MISORDERED
error as a sign we need to locate an approperiate sequence number between the 
value we sent, and the last successfully acked SEQUENCE call.

Commit 3453d5708b33efe76f40eca1c0ed60923094b971 is a clean cherry-pick to disco.

[Testcase]

This is difficult to reproduce on test systems, and has instead been verified on
a production NFS v4.1 system in a customer environment. This server is heavily 
trafficked and has a large number of different NFS clients connected to it.

I have built a test kernel that contains the above patch, and also patches for 
Bug 1842037. It is available here:

https://launchpad.net/~mruffell/+archive/ubuntu/sf241068-test

Note that the above kernel is for bionic HWE, and not explicitly disco.

Discussion about the patch validation can be found at the bottom of Bug 1842037.

On unpatched kernels, expect to see the symptoms mentioned in Impact, and on 
patched systems, everything working as intended.

[Regression Potential]

The changes are localised to NFS v4.1 only, and other versions of NFS are not 
affected. If a regression occurs, users can downgrade NFS versions to v4.0 or 
v3.x until a fix is made.

The changes only impact when connections are interrupted, and under typical blue
sky scenarios would not be invoked. 

There have been no fixup commits or commits near the requested commit in newer 
kernels, which points to this commit fixing the issue, and adopted by the 
community.

Trond Myklebust (1):
  NFSv4.1: Avoid false retries when RPC calls are interrupted

 fs/nfs/nfs4proc.c    | 105 ++++++++++++++++++++-----------------------
 fs/nfs/nfs4session.c |   5 ++-
 fs/nfs/nfs4session.h |   5 ++-
 3 files changed, 55 insertions(+), 60 deletions(-)

-- 
2.20.1