When the tail page of THP is poisoned, the head page will be poisoned too.
 And the wrong address, address of head page, will be sent with sigbus
always.

So when the poisoned page is used by Guest OS which is running on KVM,
after the address changing(hva->gpa) by qemu, the unexpected process on
Guest OS will be killed by sigbus.

What we expected is that the process using the poisoned tail page could be
killed on Guest OS, but not that the process using the healthy head page
is killed.

Since it is not good to poison the healthy page, avoid poisoning other
than the page which is really poisoned.
  (While we poison all pages in a huge page in case of hugetlb,
   we can do this for THP thanks to split_huge_page().)

Here we fix two parts:
  1. Isolate the poisoned page only to make sure
     the reported address is the address of poisoned page.
  2. make the poisoned page work as the poisoned regular page.

[akpm@linux-foundation.org: fix spello in comment]
Signed-off-by: Jin Dongming <jin.dongming@np.css.fujitsu.com>
Reviewed-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>