Linux: syncfs(2) should sync cached dirty data

While debugging problem with data not being synced on umount, we've
noticed that even syncfs(2) doesn't help to get the data written
out. It is because it doesn't actually sync any of the live znodes.

Signed-off-by: Pavel Snajdr <snajpa@snajpa.net>

include/os/linux/zfs/sys/zpl.h

@@ -49,6 +49,8 @@ extern const struct inode_operations zpl_special_inode_operations;
 extern const struct address_space_operations zpl_address_space_operations;
 extern const struct file_operations zpl_file_operations;
 extern const struct file_operations zpl_dir_file_operations;
+extern int zpl_writepages(struct address_space *mapping,
+    struct writeback_control *wbc);
 
 /* zpl_super.c */
 extern void zpl_prune_sb(uint64_t nr_to_scan, void *arg);

module/os/linux/zfs/zpl_file.c

@@ -546,7 +546,7 @@ zpl_write_cache_pages(struct address_space *mapping,
 	return (result);
 }
 
-static int
+int
 zpl_writepages(struct address_space *mapping, struct writeback_control *wbc)
 {
 	znode_t		*zp = ITOZ(mapping->host);

module/os/linux/zfs/zpl_super.c

@@ -118,11 +118,29 @@ zpl_sync_fs(struct super_block *sb, int wait)
 {
 	fstrans_cookie_t cookie;
 	cred_t *cr = CRED();
+	znode_t *zp;
+	zfsvfs_t *zfsvfs = sb->s_fs_info;
+	struct writeback_control wbc = {
+		.sync_mode = wait ? WB_SYNC_ALL : WB_SYNC_NONE,
+		.nr_to_write = LONG_MAX,
+		.range_start = 0,
+		.range_end = LLONG_MAX,
+	};
 	int error;
 
 	crhold(cr);
 	cookie = spl_fstrans_mark();
-	error = -zfs_sync(sb, wait, cr);
+	mutex_enter(&zfsvfs->z_znodes_lock);
+	for (zp = list_head(&zfsvfs->z_all_znodes); zp;
+	    zp = list_next(&zfsvfs->z_all_znodes, zp)) {
+		if (zp->z_sa_hdl)
+			error = zpl_writepages(ZTOI(zp)->i_mapping, &wbc);
+		if (error != 0)
+			break;
+	}
+	mutex_exit(&zfsvfs->z_znodes_lock);
+	if (error == 0)
+		error = -zfs_sync(sb, wait, cr);
 	spl_fstrans_unmark(cookie);
 	crfree(cr);
 	ASSERT3S(error, <=, 0);