[PATCH v2] mm, hwpoison: Try to recover from copy-on write faults

[Luck, Tony]

>> When we do return to user mode the task is going to be busy servicing
>> a SIGBUS ... so shouldn't try to touch the poison page before the
>> memory_failure() called by the worker thread cleans things up.
>
> What about an RT process on a busy system?
> The worker threads are pretty low priority.

Most tasks don't have a SIGBUS handler ... so they just die without possibility of accessing poison

If this task DOES have a SIGBUS handler, and that for some bizarre reason just does a "return"
so the task jumps back to the instruction that cause the COW then there is a 63/64
likelihood that it is touching a different cache line from the poisoned one.

In the 1/64 case ... its probably a simple store (since there was a COW, we know it was trying to
modify the page) ... so won't generate another machine check (those only happen for reads).

But maybe it is some RMW instruction ... then, if all the above options didn't happen ... we
could get another machine check from the same address. But then we just follow the usual
recovery path.

-Tony