mm/memory-failure.c: recheck PageHuge() after hugetlb page migrate successfully

commit a49ecbcd upstream.

After a successful hugetlb page migration by soft offline, the source
page will either be freed into hugepage_freelists or buddy(over-commit
page).  If page is in buddy, page_hstate(page) will be NULL.  It will
hit a NULL pointer dereference in dequeue_hwpoisoned_huge_page().

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000058
  IP: [<ffffffff81163761>] dequeue_hwpoisoned_huge_page+0x131/0x1d0
  PGD c23762067 PUD c24be2067 PMD 0
  Oops: 0000 [#1] SMP

So check PageHuge(page) after call migrate_pages() successfully.

[wujg: backport to 3.4:
 - adjust context
 - s/num_poisoned_pages/mce_bad_pages/]
Signed-off-by: Jianguo Wu <wujianguo@huawei.com>
Tested-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

mm/memory-failure.c

@@ -1447,10 +1447,18 @@ static int soft_offline_huge_page(struct page *page, int flags)
 		return ret;
 	}
 done:
-	if (!PageHWPoison(hpage))
-		atomic_long_add(1 << compound_trans_order(hpage), &mce_bad_pages);
-	set_page_hwpoison_huge_page(hpage);
-	dequeue_hwpoisoned_huge_page(hpage);
+	/* overcommit hugetlb page will be freed to buddy */
+	if (PageHuge(hpage)) {
+		if (!PageHWPoison(hpage))
+			atomic_long_add(1 << compound_trans_order(hpage),
+					&mce_bad_pages);
+		set_page_hwpoison_huge_page(hpage);
+		dequeue_hwpoisoned_huge_page(hpage);
+	} else {
+		SetPageHWPoison(page);
+		atomic_long_inc(&mce_bad_pages);
+	}
+
 	/* keep elevated page count for bad page */
 	return ret;
 }