hostfs: Convert to writepages

If we add a migrate_folio operation, we can convert the writepage
operation to writepages.  The large folio support here is illusory;
we would need to kmap each page in turn for proper support.  But we do
remove a few hidden calls to compound_head().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://patch.msgid.link/20241220051500.1919389-1-willy@infradead.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: Matthew Wilcox (Oracle)

fs/hostfs/hostfs_kern.c

@@ -410,38 +410,33 @@ static const struct file_operations hostfs_dir_fops = {
 	.fsync		= hostfs_fsync,
 };
 
-static int hostfs_writepage(struct page *page, struct writeback_control *wbc)
+static int hostfs_writepages(struct address_space *mapping,
+		struct writeback_control *wbc)
 {
-	struct address_space *mapping = page->mapping;
 	struct inode *inode = mapping->host;
-	char *buffer;
-	loff_t base = page_offset(page);
-	int count = PAGE_SIZE;
-	int end_index = inode->i_size >> PAGE_SHIFT;
-	int err;
-
-	if (page->index >= end_index)
-		count = inode->i_size & (PAGE_SIZE-1);
-
-	buffer = kmap_local_page(page);
-
-	err = write_file(HOSTFS_I(inode)->fd, &base, buffer, count);
-	if (err != count) {
-		if (err >= 0)
-			err = -EIO;
-		mapping_set_error(mapping, err);
-		goto out;
+	struct folio *folio = NULL;
+	loff_t i_size = i_size_read(inode);
+	int err = 0;
+
+	while ((folio = writeback_iter(mapping, wbc, folio, &err))) {
+		loff_t pos = folio_pos(folio);
+		size_t count = folio_size(folio);
+		char *buffer;
+		int ret;
+
+		if (count > i_size - pos)
+			count = i_size - pos;
+
+		buffer = kmap_local_folio(folio, 0);
+		ret = write_file(HOSTFS_I(inode)->fd, &pos, buffer, count);
+		kunmap_local(buffer);
+		folio_unlock(folio);
+		if (ret != count) {
+			err = ret < 0 ? ret : -EIO;
+			mapping_set_error(mapping, err);
+		}
 	}
 
-	if (base > inode->i_size)
-		inode->i_size = base;
-
-	err = 0;
-
- out:
-	kunmap_local(buffer);
-	unlock_page(page);
-
 	return err;
 }
 
@@ -506,11 +501,12 @@ static int hostfs_write_end(struct file *file, struct address_space *mapping,
 }
 
 static const struct address_space_operations hostfs_aops = {
-	.writepage 	= hostfs_writepage,
+	.writepages 	= hostfs_writepages,
 	.read_folio	= hostfs_read_folio,
 	.dirty_folio	= filemap_dirty_folio,
 	.write_begin	= hostfs_write_begin,
 	.write_end	= hostfs_write_end,
+	.migrate_folio	= filemap_migrate_folio,
 };
 
 static int hostfs_inode_update(struct inode *ino, const struct hostfs_stat *st)