mm,hwpoison: return -EHWPOISON to denote that the page has already been poisoned

When memory_failure() is called with MF_ACTION_REQUIRED on the page that
has already been hwpoisoned, memory_failure() could fail to send SIGBUS
to the affected process, which results in infinite loop of MCEs.

Currently memory_failure() returns 0 if it's called for already
hwpoisoned page, then the caller, kill_me_maybe(), could return without
sending SIGBUS to current process.  An action required MCE is raised
when the current process accesses to the broken memory, so no SIGBUS
means that the current process continues to run and access to the error
page again soon, so running into MCE loop.

This issue can arise for example in the following scenarios:

 - Two or more threads access to the poisoned page concurrently. If
   local MCE is enabled, MCE handler independently handles the MCE
   events. So there's a race among MCE events, and the second or latter
   threads fall into the situation in question.

 - If there was a precedent memory error event and memory_failure() for
   the event failed to unmap the error page for some reason, the
   subsequent memory access to the error page triggers the MCE loop
   situation.

To fix the issue, make memory_failure() return an error code when the
error page has already been hwpoisoned.  This allows memory error
handler to control how it sends signals to userspace.  And make sure
that any process touching a hwpoisoned page should get a SIGBUS even in
"already hwpoisoned" path of memory_failure() as is done in page fault
path.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Aili Yao <[email protected]>
Signed-off-by: Naoya Horiguchi <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jue Wang <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Aili Yao

mm/memory-failure.c

@@ -1253,7 +1253,7 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
 	if (TestSetPageHWPoison(head)) {
 		pr_err("Memory failure: %#lx: already hardware poisoned\n",
 		       pfn);
-		return 0;
+		return -EHWPOISON;
 	}
 
 	num_poisoned_pages_inc();
@@ -1461,6 +1461,7 @@ int memory_failure(unsigned long pfn, int flags)
 	if (TestSetPageHWPoison(p)) {
 		pr_err("Memory failure: %#lx: already hardware poisoned\n",
 			pfn);
+		res = -EHWPOISON;
 		goto unlock_mutex;
 	}