Re: nfsd deadlock, 2.6.36-rc3

[Tim Gardner]

On 09/01/2010 02:55 PM, Neil Brown wrote:
> On Wed, 1 Sep 2010 12:54:01 -0400
> "J. Bruce Fields"<bfields@xxxxxxxxxxxx>  wrote:
>
> > On Wed, Sep 01, 2010 at 09:39:55AM -0600, Tim Gardner wrote:
> > > I've been pursuing a simple reproducer for an NFS lockup that shows
> > > up under stress. There is a bunch of info (some of it extraneous) in
> > > http://bugs.launchpad.net/bugs/561210. I can reproduce it by writing
> > > loop mounted NFS exports:
> > >
> > > /etc/fstab: 127.0.0.1:/srv /mnt/srv nfs rw 0 2
> > > /etc/exports: /srv 127.0.0.1(rw,insecure,no_subtree_check)
> > >
> > > See the attached scripts test_master.sh and test_client.sh. I simply
> > > repeat './test_master.sh wait' until nfsd locks up, typically within
> > > 1-3 cycles, e.g.,
> >
> > Without looking at the dmesg and scripts carefully to confirm, one
> > possible explanation is a deadlock when the server can't allocate memory
> > required to service client requests, memory which the client itself
> > needs to free by writing back dirty pages, but can't because the server
> > isn't processing its writes.
>
> Having looked closely I'd say it is almost certainly this issue.
> nfsd thread 1266 is in zone_reclaim waiting on a page to be written out so
> the memory can be reused.
> The other nfsd threads are blocking on a mutex held by 1266.
> The dd processes are waiting for pages to be written to the server
>
> The particular page that 1266 is waiting on is almost certainly a page on an
> NFS file, so you have a cyclic deadlock.
>
> > For that reason we just don't support loopback mounts--they're OK for
> > light testing, but it would be difficult to make them completely robust
> > under load.
>
> I wonder if we could use 'containers' to partition available memory between
> 'nfsd threads' and 'everything else'??  Probably not worth the effort.
>
> NeilBrown

I'm currently working with my support folks to reproduce this using the 
exact same configuration as the customer, e.g., an NFS server (running 
as a guest on a VMWare ESX host) serving multiple gigabit clients.

I assume that is a reasonable scenario?

rtg
--
Tim Gardner tim.gardner@xxxxxxxxxxxxx
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