netdev - Re: Weird TCP hang when doing loopback NFS (wireshark traces attached)

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Date:	Sun, 1 Mar 2015 19:52:28 -0500
From:	Trond Myklebust <trond.myklebust@...marydata.com>
To:	Linux Network Devel Mailing List <netdev@...r.kernel.org>,
	Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
	Bruce James Fields <bfields@...ldses.org>
Subject: Re: Weird TCP hang when doing loopback NFS (wireshark traces attached)

Hi Bruce,

On Sun, Mar 1, 2015 at 2:14 PM, Trond Myklebust
<trond.myklebust@...marydata.com> wrote:
> Hi,
>
> When doing testing of NFSv3 loopback mounts (client and server are on
> the same IP address), I'm seeing a very reproducible hang in which the
> client stops receiving data from the server. The TCP connection is still
> marked as established, and the server appears to continue to receive and
> send data, however the client does not.
>
> So far, I've reproduced on both v4.0-rc1, and the Fedora v3.18.7 kernel.
>
> The reproducer is simply to loopback mount using NFSv3, and then run the
> 'fsx' filesystem exerciser. I'm usually able to trigger the hang with
> "fsx -N 100000 foobar".
>
> I've attached a couple of wireshark trace of a few frames just before
> and during the hang in case it jogs any memories.

This bug appears to go away when I disable the splice()-based reads by
clearing the RQ_SPLICE_OK flag.

I noticed that it always involved a combination of a READ and a
truncating SETATTR call. Are you sure that it is safe to share
pagecache pages directly with sendpage() in this way? As far as I can
tell, there is no locking to prevent them from being modified while in
the TCP send queue.

Cheers
  Trond
-- 
Trond Myklebust
Linux NFS client maintainer, PrimaryData
trond.myklebust@...marydata.com
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