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KAMEZAWA Hiroyuki found a warning message in the buffer dirtying code that
is coming from page migration caller.

WARNING: at fs/buffer.c:720 __set_page_dirty+0x330/0x360()
Call Trace:
 [<a000000100015220>] show_stack+0x80/0xa0
 [<a000000100015270>] dump_stack+0x30/0x60
 [<a000000100089ed0>] warn_on_slowpath+0x90/0xe0
 [<a0000001001f8b10>] __set_page_dirty+0x330/0x360
 [<a0000001001ffb90>] __set_page_dirty_buffers+0xd0/0x280
 [<a00000010012fec0>] set_page_dirty+0xc0/0x260
 [<a000000100195670>] migrate_page_copy+0x5d0/0x5e0
 [<a000000100197840>] buffer_migrate_page+0x2e0/0x3c0
 [<a000000100195eb0>] migrate_pages+0x770/0xe00

What was happening is that migrate_page_copy wants to transfer the PG_dirty
bit from old page to new page, so what it would do is set_page_dirty(newpage).
However set_page_dirty() is used to set the entire page dirty, wheras in
this case, only part of the page was dirty, and it also was not uptodate.

Marking the whole page dirty with set_page_dirty wo...