linux-kernel - Re: nfsd deadlock, 2.6.36-rc3

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Message-ID: <20100901211321.GC10507@fieldses.org>
Date:	Wed, 1 Sep 2010 17:13:21 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Tim Gardner <tim.gardner@...onical.com>
Cc:	Neil Brown <neilb@...e.de>, linux-nfs@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Trond.Myklebust@...app.com
Subject: Re: nfsd deadlock, 2.6.36-rc3

On Wed, Sep 01, 2010 at 03:11:23PM -0600, Tim Gardner wrote:
> On 09/01/2010 02:55 PM, Neil Brown wrote:
> >On Wed, 1 Sep 2010 12:54:01 -0400
> >"J. Bruce Fields"<bfields@...ldses.org>  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?

Assuming no VMWare problem (which I know nothing about), sure.

--b.
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