mm: khugepaged: support shmem mTHP copy

Iterate each subpage in the large folio to copy, as preparation for
supporting shmem mTHP collapse.

Link: https://lkml.kernel.org/r/222d615b7c837eabb47a238126c5fdeff8aa5283.1724140601.git.baolin.wang@linux.alibaba.comSigned-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <21cnbao@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/khugepaged.c

@@ -2056,17 +2056,22 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	index = start;
 	dst = folio_page(new_folio, 0);
 	list_for_each_entry(folio, &pagelist, lru) {
+		int i, nr_pages = folio_nr_pages(folio);
+
 		while (index < folio->index) {
 			clear_highpage(dst);
 			index++;
 			dst++;
 		}
-		if (copy_mc_highpage(dst, folio_page(folio, 0)) > 0) {
-			result = SCAN_COPY_MC;
-			goto rollback;
+
+		for (i = 0; i < nr_pages; i++) {
+			if (copy_mc_highpage(dst, folio_page(folio, i)) > 0) {
+				result = SCAN_COPY_MC;
+				goto rollback;
+			}
+			index++;
+			dst++;
 		}
-		index++;
-		dst++;
 	}
 	while (index < end) {
 		clear_highpage(dst);