kswapd blocked trying to free dirty NFS pages on RHEL 5

Issue

• RHEL 5 NFS client freezes under heavy NFS load
• NFS Client running on RHEL 5 hung temporarily for around 5-7 minutes while performing a Database backup to an NFS mount.
• Slow performance when copying a file between two mount points on a Netapp Filer
• Lock-up writing to nfs share
• System was rebooted. Message "task kswapd0:1503 blocked for more than 120 seconds" seen in messages file.

INFO: task kswapd0:670 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kswapd0       D ffff810001004420     0   670    236           671   669 (L-TLB)
 ffff8103475399e0 0000000000000046 ffff81010b8572c0 ffff8102c452d000
 000000000000000c 000000000000000a ffff8106474a6100 ffffffff80310b60
 00001dc1f306c3a7 0000000000007f6c ffff8106474a62e8 00000000800de2c3
Call Trace:
 [<ffffffff8006ec4e>] do_gettimeofday+0x40/0x90
 [<ffffffff89337941>] :nfs:nfs_wait_bit_uninterruptible+0x0/0xd
 [<ffffffff800637ca>] io_schedule+0x3f/0x67
 [<ffffffff8933794a>] :nfs:nfs_wait_bit_uninterruptible+0x9/0xd
 [<ffffffff800639f6>] __wait_on_bit+0x40/0x6e
 [<ffffffff89337941>] :nfs:nfs_wait_bit_uninterruptible+0x0/0xd
 [<ffffffff80063a90>] out_of_line_wait_on_bit+0x6c/0x78
 [<ffffffff800a28e2>] wake_bit_function+0x0/0x23
 [<ffffffff8933b30a>] :nfs:nfs_wait_on_requests_locked+0x70/0xca
 [<ffffffff8933c340>] :nfs:nfs_sync_inode_wait+0x60/0x1db
 [<ffffffff893322c8>] :nfs:nfs_release_page+0x2c/0x4d
 [<ffffffff800cd32c>] shrink_inactive_list+0x511/0x8d8
 [<ffffffff800cc81f>] isolate_lru_pages+0x98/0xbf
 [<ffffffff80048014>] __pagevec_release+0x19/0x22
 [<ffffffff800cccfa>] shrink_active_list+0x4b4/0x4c4
 [<ffffffff800131c7>] shrink_zone+0x127/0x18d
 [<ffffffff80057c0a>] kswapd+0x33d/0x495
 [<ffffffff800a28b4>] autoremove_wake_function+0x0/0x2e
 [<ffffffff800578cd>] kswapd+0x0/0x495
 [<ffffffff800a269c>] keventd_create_kthread+0x0/0xc4
 [<ffffffff80032996>] kthread+0xfe/0x132
 [<ffffffff8005dfb1>] child_rip+0xa/0x11
 [<ffffffff800a269c>] keventd_create_kthread+0x0/0xc4
 [<ffffffff80032898>] kthread+0x0/0x132
 [<ffffffff8005dfa7>] child_rip+0x0/0x11