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/20231218135837.3310403-3-willy@infradead.org
Fixes: 7af446a ("HWPOISON, hugetlb: enable error handling path for hugepage")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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