From: Hugh Dickins <hugh@veritas.com>

PageDirty BUG_ON in __remove_inode_page is, and always has been, unsafe for
SMP or preemption: truncation may be racing against unmapping's
set_page_dirty in zap_pte_range (amongst a few other possibilities).
!PageUptodate error in __set_page_dirty_buffers is unsafe then too.