Re: nfsd deadlock, 2.6.36-rc3

[Tim Gardner <tim.gardner@canonical.com>]

On 09/01/2010 02:55 PM, Neil Brown wrote:
> On Wed, 1 Sep 2010 12:54:01 -0400
> "J. Bruce Fields"<bfields@fieldses.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?

rtg
-- 
Tim Gardner tim.gardner@canonical.com