In xfs_vm_write_failed(), we evaluate the block_offset of pos with
PAGE_MASK which is an unsigned long.  That is fine on 64-bit platforms
regardless of whether the request pos is 32-bit or 64-bit.  However, on
32-bit platforms the value is 0xfffff000 and so the high 32 bits in it
will be masked off with (pos & PAGE_MASK) for a 64-bit pos.

As a result, the evaluated block_offset is incorrect which will cause
this failure ASSERT(block_offset + from == pos); and potentially pass
the wrong block to xfs_vm_kill_delalloc_range().

In this case, we can get a kernel panic if CONFIG_XFS_DEBUG is enabled:

XFS: Assertion failed: block_offset + from == pos, file: fs/xfs/xfs_aops.c, line: 1504

------------[ cut here ]------------
 kernel BUG at fs/xfs/xfs_message.c:100!
 invalid opcode: 0000 [#1] SMP
 ........
 Pid: 4057, comm: mkfs.xfs Tainted: G           O 3.9.0-rc2 #1
 EIP: 0060:[<f94a7e8b>] EFLAGS: 00010282 CPU: 0
 EIP is at assfail+0x2b/0x30 [xfs]
 EAX: 00000056 EBX: f6ef28a0 ECX: 00000007 EDX: f57d22a4
 ESI: 1c2fb000 EDI: 00000000 EBP: ea6b5d30 ESP: ea6b5d1c
 DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
 CR0: 8005003b CR2: 094f3ff4 CR3: 2bcb4000 CR4: 000006f0
 DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
 DR6: ffff0ff0 DR7: 00000400
 Process mkfs.xfs (pid: 4057, ti=ea6b4000 task=ea5799e0 task.ti=ea6b4000)
 Stack:
 00000000 f9525c48 f951fa80 f951f96b 000005e4 ea6b5d7c f9494b34 c19b0ea2
 00000066 f3d6c620 c19b0ea2 00000000 e9a91458 00001000 00000000 00000000
 00000000 c15c7e89 00000000 1c2fb000 00000000 00000000 1c2fb000 00000080
 Call Trace:
 [<f9494b34>] xfs_vm_write_failed+0x74/0x1b0 [xfs]
 [<c15c7e89>] ? printk+0x4d/0x4f
 [<f9494d7d>] xfs_vm_write_begin+0x10d/0x170 [xfs]
 [<c110a34c>] generic_file_buffered_write+0xdc/0x210
 [<f949b669>] xfs_file_buffered_aio_write+0xf9/0x190 [xfs]
 [<f949b7f3>] xfs_file_aio_write+0xf3/0x160 [xfs]
 [<c115e504>] do_sync_write+0x94/0xd0
 [<c115ed1f>] vfs_write+0x8f/0x160
 [<c115e470>] ? wait_on_retry_sync_kiocb+0x50/0x50
 [<c115f017>] sys_write+0x47/0x80
 [<c15d860d>] sysenter_do_call+0x12/0x28
 .............
 EIP: [<f94a7e8b>] assfail+0x2b/0x30 [xfs] SS:ESP 0068:ea6b5d1c
 ---[ end trace cdd9af4f4ecab42f ]---
 Kernel panic - not syncing: Fatal exception

In order to avoid this, we can evaluate the block_offset of the start
of the page by using shifts rather than masks the mismatch problem.

Thanks Dave Chinner for help finding and fixing this bug.
Reported-by: NMichael L. Semon <mlsemon35@gmail.com>
Reviewed-by: NDave Chinner <david@fromorbit.com>
Reviewed-by: NMark Tinguely <tinguely@sgi.com>
Signed-off-by: NJie Liu <jeff.liu@oracle.com>
Signed-off-by: NBen Myers <bpm@sgi.com>