vmscan: convert page buffer handling to use folios

This mostly just removes calls to compound_head() although nr_reclaimed
should be incremented by the number of pages, not just 1.

Link: https://lkml.kernel.org/r/20220504182857.4013401-11-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmscan.c

@@ -1859,38 +1859,40 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 		}
 
 		/*
-		 * If the page has buffers, try to free the buffer mappings
-		 * associated with this page. If we succeed we try to free
-		 * the page as well.
+		 * If the folio has buffers, try to free the buffer
+		 * mappings associated with this folio. If we succeed
+		 * we try to free the folio as well.
 		 *
-		 * We do this even if the page is PageDirty().
-		 * try_to_release_page() does not perform I/O, but it is
-		 * possible for a page to have PageDirty set, but it is actually
-		 * clean (all its buffers are clean).  This happens if the
-		 * buffers were written out directly, with submit_bh(). ext3
-		 * will do this, as well as the blockdev mapping.
-		 * try_to_release_page() will discover that cleanness and will
-		 * drop the buffers and mark the page clean - it can be freed.
+		 * We do this even if the folio is dirty.
+		 * filemap_release_folio() does not perform I/O, but it
+		 * is possible for a folio to have the dirty flag set,
+		 * but it is actually clean (all its buffers are clean).
+		 * This happens if the buffers were written out directly,
+		 * with submit_bh(). ext3 will do this, as well as
+		 * the blockdev mapping.  filemap_release_folio() will
+		 * discover that cleanness and will drop the buffers
+		 * and mark the folio clean - it can be freed.
 		 *
-		 * Rarely, pages can have buffers and no ->mapping.  These are
-		 * the pages which were not successfully invalidated in
-		 * truncate_cleanup_page().  We try to drop those buffers here
-		 * and if that worked, and the page is no longer mapped into
-		 * process address space (page_count == 1) it can be freed.
-		 * Otherwise, leave the page on the LRU so it is swappable.
+		 * Rarely, folios can have buffers and no ->mapping.
+		 * These are the folios which were not successfully
+		 * invalidated in truncate_cleanup_folio().  We try to
+		 * drop those buffers here and if that worked, and the
+		 * folio is no longer mapped into process address space
+		 * (refcount == 1) it can be freed.  Otherwise, leave
+		 * the folio on the LRU so it is swappable.
 		 */
-		if (page_has_private(page)) {
-			if (!try_to_release_page(page, sc->gfp_mask))
+		if (folio_has_private(folio)) {
+			if (!filemap_release_folio(folio, sc->gfp_mask))
 				goto activate_locked;
-			if (!mapping && page_count(page) == 1) {
-				unlock_page(page);
-				if (put_page_testzero(page))
+			if (!mapping && folio_ref_count(folio) == 1) {
+				folio_unlock(folio);
+				if (folio_put_testzero(folio))
 					goto free_it;
 				else {
 					/*
 					 * rare race with speculative reference.
 					 * the speculative reference will free
-					 * this page shortly, so we may
+					 * this folio shortly, so we may
 					 * increment nr_reclaimed here (and
 					 * leave it off the LRU).
 					 */