mm: memory-failure: avoid false hwpoison page mapped error info

folio->_mapcount is overloaded in SLAB, so folio_mapped() has to be done
after folio_test_slab() is checked. Otherwise slab folio might be treated
as a mapped folio leading to false 'Someone maps the hwpoison page' error
info.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 230ac71 ("mm/hwpoison: don't try to unpoison containment-failed pages")
Signed-off-by: Miaohe Lin <[email protected]>
Reviewed-by: Matthew Wilcox (Oracle) <[email protected]>
Acked-by: Naoya Horiguchi <[email protected]>
Cc: Kefeng Wang <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: MiaoheLin

mm/memory-failure.c

@@ -2499,6 +2499,13 @@ int unpoison_memory(unsigned long pfn)
 		goto unlock_mutex;
 	}
 
+	if (folio_test_slab(folio) || PageTable(&folio->page) || folio_test_reserved(folio))
+		goto unlock_mutex;
+
+	/*
+	 * Note that folio->_mapcount is overloaded in SLAB, so the simple test
+	 * in folio_mapped() has to be done after folio_test_slab() is checked.
+	 */
 	if (folio_mapped(folio)) {
 		unpoison_pr_info("Unpoison: Someone maps the hwpoison page %#lx\n",
 				 pfn, &unpoison_rs);
@@ -2511,9 +2518,6 @@ int unpoison_memory(unsigned long pfn)
 		goto unlock_mutex;
 	}
 
-	if (folio_test_slab(folio) || PageTable(&folio->page) || folio_test_reserved(folio))
-		goto unlock_mutex;
-
 	ghp = get_hwpoison_page(p, MF_UNPOISON);
 	if (!ghp) {
 		if (PageHuge(p)) {