mm/hwpoison: don't try to unpoison containment-failed pages

memory_failure() can be called at any page at any time, which means that
we can't eliminate the possibility of containment failure.  In such case
the best option is to leak the page intentionally (and never touch it
later.)

We have an unpoison function for testing, and it cannot handle such
containment-failed pages, which results in kernel panic (visible with
various calltraces.) So this patch suggests that we limit the
unpoisonable pages to properly contained pages and ignore any other
ones.

Testers are recommended to keep in mind that there're un-unpoisonable
pages when writing test programs.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Tested-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory-failure.c

@@ -1445,6 +1445,22 @@ int unpoison_memory(unsigned long pfn)
 		return 0;
 	}
 
+	if (page_count(page) > 1) {
+		pr_info("MCE: Someone grabs the hwpoison page %#lx\n", pfn);
+		return 0;
+	}
+
+	if (page_mapped(page)) {
+		pr_info("MCE: Someone maps the hwpoison page %#lx\n", pfn);
+		return 0;
+	}
+
+	if (page_mapping(page)) {
+		pr_info("MCE: the hwpoison page has non-NULL mapping %#lx\n",
+			pfn);
+		return 0;
+	}
+
 	/*
 	 * unpoison_memory() can encounter thp only when the thp is being
 	 * worked by memory_failure() and the page lock is not held yet.