mm/memory-failure: try to send SIGBUS even if unmap failed

Patch series "Enhance soft hwpoison handling and injection", v4.

This series is aimed at the following enhancements:

- Let one hwpoison injector, that is, madvise(MADV_HWPOISON) to behave
  more like as if a real UE occurred.  Because the other two injectors
  such as hwpoison-inject and the 'einj' on x86 can't, and it seems to me
  we need a better simulation to real UE scenario.
- For years, if the kernel is unable to unmap a hwpoisoned page, it send
  a SIGKILL instead of SIGBUS to prevent user process from potentially
  accessing the page again.  But in doing so, the user process also lose
  important information: vaddr, for recovery.  Fortunately, the kernel
  already has code to kill process re-accessing a hwpoisoned page, so
  remove the '!unmap_success' check.
- Right now, if a thp page under GUP longterm pin is hwpoisoned, and
  kernel cannot split the thp page, memory-failure simply ignores the UE
  and returns.  That's not ideal, it could deliver a SIGBUS with useful
  information for userspace recovery.


This patch (of 5):

For years when it comes down to kill a process due to hwpoison, a SIGBUS
is delivered only if unmap has been successful.  Otherwise, a SIGKILL is
delivered.  And the reason for that is to prevent the involved process
from accessing the hwpoisoned page again.

Since then a lot has changed, a hwpoisoned page is marked and upon being
re-accessed, the memory-failure handler invokes kill_accessing_process()
to kill the process immediately.  So let's take out the '!unmap_success'
factor and try to deliver SIGBUS if possible.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Jane Chu <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Acked-by: Miaohe Lin <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Oscar Salvador <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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