I am facing this one too - not using nfs, but a simple dovecot imap server (but using xen):

[2831529.979439] Unable to handle kernel paging request at ffff88005e800008 RIP: 
[2831529.979447]  [<ffffffff80271fc6>] iov_iter_advance+0x66/0x80
[2831529.979458] PGD 1f6b067 PUD 216d067 PMD 2262067 PTE 0
[2831529.979462] Oops: 0000 [1] SMP 
[2831529.979465] CPU 0 
[2831529.979467] Modules linked in: af_packet ipv6 evdev ext3 jbd mbcache raid10 raid456 async_xor async_memcpy async_tx xor raid1 raid0 multipath linear md_mod dm_mirror dm_snapshot dm_mod fuse
[2831529.979482] Pid: 9629, comm: imap Not tainted 2.6.24-18-xen #1
[2831529.979484] RIP: e030:[<ffffffff80271fc6>]  [<ffffffff80271fc6>] iov_iter_advance+0x66/0x80
[2831529.979488] RSP: e02b:ffff8800308e3b10  EFLAGS: 00010246
[2831529.979490] RAX: 0000000000000000 RBX: 00000000000003f2 RCX: 0000000000000000
[2831529.979492] RDX: 0000000000000000 RSI: 00000000000003f2 RDI: ffff8800308e3ba8
[2831529.979494] RBP: 00000000000003f2 R08: 0000000000000000 R09: 0000000000000000
[2831529.979496] R10: ffff88005e800000 R11: 0000000000000000 R12: 00000000001cab54
[2831529.979498] R13: 00000000000004ac R14: ffff88005ce7e598 R15: 0000000000000b54
[2831529.979501] FS:  00007ff0dd2e36e0(0000) GS:ffffffff805c6000(0000) knlGS:0000000000000000
[2831529.979503] CS:  e033 DS: 0000 ES: 0000
[2831529.979505] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[2831529.979507] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000000
[2831529.979510] Process imap (pid: 9629, threadinfo ffff8800308e2000, task ffff88005d9e8040)
[2831529.979512] Stack:  ffffffff80273f9e ffff8800096bfe80 0000000000000026 0000000000000026
[2831529.979517]  ffff8800308e3db8 00000000001cab54 ffff8800308e3d38 ffff88005007c840
[2831529.979520]  ffff88005ce7e598 ffffffff880bcac0 ffff88005ce7e488 0000000000000000
[2831529.979524] Call Trace:
[2831529.979528]  [<ffffffff80273f9e>] generic_file_buffered_write+0x1de/0x6e0
[2831529.979532]  [<ffffffff8023ab0e>] current_fs_time+0x1e/0x30
[2831529.979536]  [<ffffffff802746ef>] __generic_file_aio_write_nolock+0x24f/0x400
[2831529.979539]  [<ffffffff80461fd9>] unix_stream_recvmsg+0x269/0x5d0
[2831529.979543]  [<ffffffff80274904>] generic_file_aio_write+0x64/0xd0
[2831529.979552]  [<ffffffff880a9663>] :ext3:ext3_file_write+0x23/0xc0
[2831529.979558]  [<ffffffff880a9640>] :ext3:ext3_file_write+0x0/0xc0
[2831529.979561]  [<ffffffff8029d6bb>] do_sync_readv_writev+0xcb/0x110
[2831529.979565]  [<ffffffff8024cc20>] autoremove_wake_function+0x0/0x30
[2831529.979568]  [<ffffffff8029d8f9>] do_sync_read+0xd9/0x120
[2831529.979571]  [<ffffffff8029a30c>] __kmalloc+0x13c/0x160
[2831529.979574]  [<ffffffff8029de5d>] do_readv_writev+0xfd/0x230
[2831529.979577]  [<ffffffff8046ffea>] schedule_timeout+0x5a/0xd0
[2831529.979579]  [<ffffffff80470349>] mutex_lock+0x9/0x20
[2831529.979582]  [<ffffffff802ce5e5>] sys_epoll_wait+0x245/0x4e0
[2831529.979586]  [<ffffffff8029e4d3>] sys_writev+0x53/0xc0
[2831529.979589]  [<ffffffff8020c698>] system_call+0x68/0x6d
[2831529.979591]  [<ffffffff8020c630>] system_call+0x0/0x6d
[2831529.979593] 
[2831529.979594] 
[2831529.979595] Code: 49 8b 52 08 49 89 d3 eb c4 4c 89 17 4c 89 4f 10 eb 99 0f 1f 
[2831529.979605] RIP  [<ffffffff80271fc6>] iov_iter_advance+0x66/0x80
[2831529.979608]  RSP <ffff8800308e3b10>
[2831529.979610] CR2: ffff88005e800008
[2831529.979614] ---[ end trace efc314f13325de04 ]---