mm/memory-failure: check the mapcount of the precise page

A process may map only some of the pages in a folio, and might be missed
if it maps the poisoned page but not the head page.  Or it might be
unnecessarily hit if it maps the head page, but not the poisoned page.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 7af446a ("HWPOISON, hugetlb: enable error handling path for hugepage")
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Matthew Wilcox (Oracle)

mm/memory-failure.c

@@ -1570,7 +1570,7 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
 	 * This check implies we don't kill processes if their pages
 	 * are in the swap cache early. Those are always late kills.
 	 */
-	if (!page_mapped(hpage))
+	if (!page_mapped(p))
 		return true;
 
 	if (PageSwapCache(p)) {
@@ -1621,10 +1621,10 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
 		try_to_unmap(folio, ttu);
 	}
 
-	unmap_success = !page_mapped(hpage);
+	unmap_success = !page_mapped(p);
 	if (!unmap_success)
 		pr_err("%#lx: failed to unmap page (mapcount=%d)\n",
-		       pfn, page_mapcount(hpage));
+		       pfn, page_mapcount(p));
 
 	/*
 	 * try_to_unmap() might put mlocked page in lru cache, so call