We're also seing this kernel bug in relation to NFS .. (not using XEN at all);

Aug  4 14:56:00 ko kernel: [263156.166854] PGD 8063 PUD 9063 PMD 0 
Aug  4 14:56:00 ko kernel: [263156.166905] CPU 2 
Aug  4 14:56:00 ko kernel: [263156.166926] Modules linked in: nfsd auth_rpcgss exportfs nfs lockd nfs_acl sunrpc autofs4 iptable_filter ip_tables x_
tables parport_pc lp parport loop joydev ipv6 psmouse evdev serio_raw pcspkr jedec_probe cfi_probe gen_probe mtd chipreg map_funcs i2c_nforce2 i2c_c
ore button shpchp pci_hotplug k8temp ext3 jbd mbcache pata_amd pata_acpi sr_mod cdrom usb_storage libusual usbhid hid sg sd_mod mptsas mptscsih mptb
ase scsi_transport_sas qla2xxx scsi_transport_fc e1000 scsi_tgt ata_generic ehci_hcd ohci_hcd libata scsi_mod usbcore thermal processor fan fbcon tileblit font bitblit softcursor fuse
Aug  4 14:56:00 ko kernel: [263156.167218] Pid: 7150, comm: nfsd Not tainted 2.6.24-19-server #1
Aug  4 14:56:00 ko kernel: [263156.167246] RIP: 0010:[iov_iter_advance+0x66/0x80]  [iov_iter_advance+0x66/0x80] iov_iter_advance+0x66/0x80
Aug  4 14:56:00 ko kernel: [263156.167292] RSP: 0018:ffff8101fa8478e8  EFLAGS: 00010246
Aug  4 14:56:00 ko kernel: [263156.167317] RAX: 0000000000000000 RBX: 0000000000001000 RCX: 0000000000000000
Aug  4 14:56:00 ko kernel: [263156.167360] RDX: 0000000000000000 RSI: 0000000000001000 RDI: ffff8101fa847980
Aug  4 14:56:00 ko kernel: [263156.167402] RBP: 0000000000001000 R08: 0000000000000000 R09: 0000000000000000
Aug  4 14:56:00 ko kernel: [263156.167444] R10: ffff810004000000 R11: 0000000000000000 R12: 00000005ba1f8000
Aug  4 14:56:00 ko kernel: [263156.167487] R13: 0000000000001000 R14: ffff8101760241d8 R15: 0000000000000000
Aug  4 14:56:00 ko kernel: [263156.167530] FS:  00007f9a5e8106e0(0000) GS:ffff8101fb042380(0000) knlGS:0000000000000000
Aug  4 14:56:00 ko kernel: [263156.167574] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
Aug  4 14:56:00 ko kernel: [263156.167600] CR2: ffff810004000008 CR3: 0000000055597000 CR4: 00000000000006e0
Aug  4 14:56:00 ko kernel: [263156.167643] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
Aug  4 14:56:00 ko kernel: [263156.167686] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Aug  4 14:56:00 ko kernel: [263156.167730] Process nfsd (pid: 7150, threadinfo ffff8101fa846000, task ffff8101fb8947e0)
Aug  4 14:56:00 ko kernel: [263156.167774] Stack:  ffffffff80286a9e ffff8101e8df4600 ffff81033ec88680 0000000100000000
Aug  4 14:56:00 ko kernel: [263156.167823]  ffff8101fa847b90 00000005ba0f9000 ffff8101fa847b10 ffff8101e8df4600
Aug  4 14:56:00 ko kernel: [263156.167871]  ffff8101760241d8 ffffffff882559a0 ffff8101760240c8 ffffffff882559a0
Aug  4 14:56:00 ko kernel: [263156.167903] Call Trace:
Aug  4 14:56:00 ko kernel: [263156.167940]  [generic_file_buffered_write+0x1de/0x6b0] generic_file_buffered_write+0x1de/0x6b0
Aug  4 14:56:00 ko kernel: [263156.167979]  [__generic_file_aio_write_nolock+0x24f/0x400] __generic_file_aio_write_nolock+0x24f/0x400
Aug  4 14:56:00 ko kernel: [263156.168028]  [ext3:generic_file_aio_write+0x64/0x450] generic_file_aio_write+0x64/0xd0
Aug  4 14:56:00 ko kernel: [263156.168069]  [ext3:ext3_file_write+0x23/0xc0] :ext3:ext3_file_write+0x23/0xc0
Aug  4 14:56:00 ko kernel: [263156.168100]  [ext3:ext3_file_write+0x0/0xc0] :ext3:ext3_file_write+0x0/0xc0
Aug  4 14:56:00 ko kernel: [263156.168129]  [do_sync_readv_writev+0xcb/0x110] do_sync_readv_writev+0xcb/0x110
Aug  4 14:56:00 ko kernel: [263156.168161]  [__alloc_pages+0x9d/0x3d0] __alloc_pages+0x9d/0x3d0
Aug  4 14:56:00 ko kernel: [263156.168189]  [<ffffffff80254260>] autoremove_wake_function+0x0/0x30
Aug  4 14:56:00 ko kernel: [263156.168222]  [ext3:__get_free_pages+0x1b/0x1320] __get_free_pages+0x1b/0x40
Aug  4 14:56:00 ko kernel: [263156.168251]  [do_readv_writev+0xfd/0x230] do_readv_writev+0xfd/0x230
Aug  4 14:56:00 ko kernel: [263156.168293]  [<ffffffff8843c9e2>] :nfsd:nfsd_setuser+0x82/0x1f0
Aug  4 14:56:00 ko kernel: [263156.168327]  [<ffffffff88434f6c>] :nfsd:nfsd_setuser_and_check_port+0x5c/0x70
Aug  4 14:56:00 ko kernel: [263156.168379]  [<ffffffff88436790>] :nfsd:nfsd_vfs_write+0xf0/0x3d0
Aug  4 14:56:00 ko kernel: [263156.168410]  [__dentry_open+0x1a7/0x200] __dentry_open+0x1a7/0x200
Aug  4 14:56:00 ko kernel: [263156.168443]  [<ffffffff88436fbc>] :nfsd:nfsd_open+0x12c/0x1e0
Aug  4 14:56:00 ko kernel: [263156.168477]  [<ffffffff8843731d>] :nfsd:nfsd_write+0x10d/0x120
Aug  4 14:56:00 ko kernel: [263156.168512]  [<ffffffff8843f86b>] :nfsd:nfsd3_proc_write+0x10b/0x160
Aug  4 14:56:00 ko kernel: [263156.168547]  [<ffffffff88432271>] :nfsd:nfsd_dispatch+0xb1/0x240
Aug  4 14:56:00 ko kernel: [263156.168597]  [<ffffffff8838bdad>] :sunrpc:svc_process+0x47d/0x7e0
Aug  4 14:56:00 ko kernel: [263156.168627]  [<ffffffff802362c0>] default_wake_function+0x0/0x10
Aug  4 14:56:00 ko kernel: [263156.168658]  [__down_read+0x12/0xb1] __down_read+0x12/0xb1
Aug  4 14:56:00 ko kernel: [263156.168689]  [<ffffffff88432810>] :nfsd:nfsd+0x0/0x2e0
Aug  4 14:56:00 ko kernel: [263156.168718]  [<ffffffff8843299f>] :nfsd:nfsd+0x18f/0x2e0
Aug  4 14:56:00 ko kernel: [263156.168748]  [child_rip+0xa/0x12] child_rip+0xa/0x12
Aug  4 14:56:00 ko kernel: [263156.168777]  [<ffffffff88432810>] :nfsd:nfsd+0x0/0x2e0
Aug  4 14:56:00 ko kernel: [263156.168810]  [<ffffffff88432810>] :nfsd:nfsd+0x0/0x2e0
Aug  4 14:56:00 ko kernel: [263156.168835]  [child_rip+0x0/0x12] child_rip+0x0/0x12
Aug  4 14:56:00 ko kernel: [263156.168861] 
Aug  4 14:56:00 ko kernel: [263156.168879] 
Aug  4 14:56:00 ko kernel: [263156.168879] Code: 49 8b 52 08 49 89 d3 eb c4 4c 89 17 4c 89 4f 10 eb 99 0f 1f 
Aug  4 14:56:00 ko kernel: [263156.168991]  RSP <ffff8101fa8478e8>
Aug  4 14:56:00 ko kernel: [263156.169317] ---[ end trace 016407414603cb8e ]---

This is fixed by the most recent patch to 2.6.26.1: 
http://www.kernel.org/pub/linux/kernel/v2.6/ChangeLog-2.6.26.1
commit a6b79bb88e6682d2739aa5b4db7184038bbb32ce