What are processes "[123.45.78.901-ma]" on linux, where the number is an NFS server's IP address?

Question

I am seeing processes on Linux with the name "[123.45.78.901-ma]", where the number is the IP address of our NFS server. Does anyone know what these processes are, where I can find out information about them, or a good keyword to search the web with?

Background:

I see the processes on a machine where something funky is going on with NFS - something seems to be stuck in a loop making a huge number of NFS requests (read, renew, and authrefrsh), but all obvious candidates (like user programs) that might be doing so have been killed.

A large number of [123.45.78.901-ma] processes exist (for brief periods of time) on the machine in question, but not (at least not for long enough that I have detected them) on machines that are acting normally.

So it would appear that these processes are involved somehow in the problem, but I don't know how.

I found them while looking for processes in "uninterruptible sleep", via

while [ 1 ] ; do ps -w -w -e -o pid,state,tname,time,args| grep ' D ' | grep -v grep; sleep 0.1; done

Details about the system:

• Red Hat Enterprise Linux Workstation release 6.3 (Santiago)

• Linux [...] 2.6.32-279.5.1.el6.x86_64 #1 SMP Tue Jul 24 13:57:35 EDT 2012 x86_64 x86_64 x86_64 GNU/Linux

Thanks in advance!

Answer 1

Those are kernel threads which are likely to be NFS mount helpers, which only appear when a new NFS mount is being mounted on a client.

You can see what they are sleeping on if you can look in /proc/<pid>/stack when they are in the uninterruptible sleep state, which gives a backtrace of what they are doing. This will require root privileges though, which by the sounds of your comments on the other answer isn't something you have in this case.

If you are seeing these kernel threads on this host but not others which are set up in a similar way then this implies that the RPC call performance on this host is lower than the other hosts for some reason. Perhaps there are other network problems which are causing this to show up?

For info, I had a similar finding when investigating NFS mounts hanging on otherwise-working hosts. In my case it appeared to be due to a kernel bug in the particular kernel version the hosts were running at the time, causing the [123.456.78.90-ma] kernel threads to become permanently stuck in the uninterruptible sleep state.

Answer 2

On Linux, ps reports kernel threads in square brackets. These are not "processes" in the normal sense of that word. That is, there is not an executable loaded from disk to start them, they aren't owned by a normal user, etc. They're just one of the many things the kernel has going on in the background.

For that reason, the name shown by ps doesn't have to correspond to any file on your hard disk. (In the case of zombie processes, though, it does.)