Comment # 5 on bug 921494 from Gang He

The problem is still reproduced on sles12sp2,I will verify the patch.
If it is effective, I will back-port the patch. 

ocfs2te+ 18245  0.5  0.0   6108  1348 pts/0    D+   14:33   0:08 mmap_truncate
-c 20 -s 300 /mnt/shared/mmaptruncate.txt
ocfs2te+ 18246  0.8  0.0   6108    84 pts/0    D+   14:33   0:11 mmap_truncate
-c 20 -s 300 /mnt/shared/mmaptruncate.txt
root     18249  0.0  0.0      0     0 ?        S    14:33   0:00 [kworker/3:2]
root     18376  0.0  0.0      0     0 ?        S<   14:35   0:00 [kworker/2:0H]
root     18925  0.0  0.0      0     0 ?        S    14:45   0:00 [kworker/3:0]
root     19371  0.0  0.0      0     0 ?        S<   14:52   0:00 [kworker/2:2H]
root     19584  0.0  0.1  35592  3268 pts/2    R+   14:56   0:00 ps aux
sles12sp2-nd1:/usr/local/ocfs2-test/bin # cat /proc/18245/stack
[<ffffffff81183b19>] __lock_page+0xa9/0xb0
[<ffffffffa0573f8b>] ocfs2_write_begin_nolock+0x131b/0x1830 [ocfs2]
[<ffffffffa059902b>] ocfs2_page_mkwrite+0x1ab/0x260 [ocfs2]
[<ffffffff811aea89>] do_page_mkwrite+0x69/0xb0
[<ffffffff811b14cd>] handle_pte_fault+0xfd/0x14f0
[<ffffffff811b37ae>] handle_mm_fault+0x29e/0x550
[<ffffffff810645ba>] __do_page_fault+0x18a/0x410
[<ffffffff810648ec>] trace_do_page_fault+0x3c/0x120
[<ffffffff815e3818>] async_page_fault+0x28/0x30
[<ffffffffffffffff>] 0xffffffffffffffff
sles12sp2-nd1:/usr/local/ocfs2-test/bin # cat /proc/18246/stack
[<ffffffff8131b4c3>] call_rwsem_down_write_failed+0x13/0x20
[<ffffffffa0589717>] ocfs2_truncate_file+0x127/0x6c0 [ocfs2]
[<ffffffffa058c1a8>] ocfs2_setattr+0x698/0xa90 [ocfs2]
[<ffffffffa05843f3>] ocfs2_inode_unlock+0x33/0x80 [ocfs2]
[<ffffffff8121587e>] notify_change+0x1ae/0x380
[<ffffffff811f8bfe>] do_truncate+0x5e/0x90
[<ffffffff811f8f58>] do_sys_ftruncate.constprop.11+0x108/0x160
[<ffffffff815e142e>] entry_SYSCALL_64_fastpath+0x12/0x6d
[<ffffffffffffffff>] 0xffffffffffffffff