zloop: advertise a volatile write cache

Zloop is file system backed and thus needs to sync the underlying file
system to persist data.  Set BLK_FEAT_WRITE_CACHE so that the block
layer actually send flush commands, and fix the flush implementation
as sync_filesystem requires s_umount to be held and the code currently
misses that.

Fixes: eb0570c ("block: new zoned loop block device driver")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Christoph Hellwig

drivers/block/zloop.c

@@ -542,6 +542,21 @@ static void zloop_rw(struct zloop_cmd *cmd)
 	zloop_put_cmd(cmd);
 }
 
+/*
+ * Sync the entire FS containing the zone files instead of walking all files.
+ */
+static int zloop_flush(struct zloop_device *zlo)
+{
+	struct super_block *sb = file_inode(zlo->data_dir)->i_sb;
+	int ret;
+
+	down_read(&sb->s_umount);
+	ret = sync_filesystem(sb);
+	up_read(&sb->s_umount);
+
+	return ret;
+}
+
 static void zloop_handle_cmd(struct zloop_cmd *cmd)
 {
 	struct request *rq = blk_mq_rq_from_pdu(cmd);
@@ -562,11 +577,7 @@ static void zloop_handle_cmd(struct zloop_cmd *cmd)
 		zloop_rw(cmd);
 		return;
 	case REQ_OP_FLUSH:
-		/*
-		 * Sync the entire FS containing the zone files instead of
-		 * walking all files
-		 */
-		cmd->ret = sync_filesystem(file_inode(zlo->data_dir)->i_sb);
+		cmd->ret = zloop_flush(zlo);
 		break;
 	case REQ_OP_ZONE_RESET:
 		cmd->ret = zloop_reset_zone(zlo, rq_zone_no(rq));
@@ -981,7 +992,8 @@ static int zloop_ctl_add(struct zloop_options *opts)
 	struct queue_limits lim = {
 		.max_hw_sectors		= SZ_1M >> SECTOR_SHIFT,
 		.chunk_sectors		= opts->zone_size,
-		.features		= BLK_FEAT_ZONED,
+		.features		= BLK_FEAT_ZONED | BLK_FEAT_WRITE_CACHE,
+
 	};
 	unsigned int nr_zones, i, j;
 	struct zloop_device *zlo;