nbd: implement the WRITE_ZEROES command

The NBD protocol defines a message for zeroing out a region of an export

Add support to the kernel driver for that message.
Signed-off-by: Wouter Verhelst <w@uter.be>
Cc: Eric Blake <eblake@redhat.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20240812133032.115134-3-w@uter.beSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -363,6 +363,8 @@ static int __nbd_set_size(struct nbd_device *nbd, loff_t bytesize,
 	}
 	if (nbd->config->flags & NBD_FLAG_ROTATIONAL)
 		lim.features |= BLK_FEAT_ROTATIONAL;
+	if (nbd->config->flags & NBD_FLAG_SEND_WRITE_ZEROES)
+		lim.max_write_zeroes_sectors = UINT_MAX >> SECTOR_SHIFT;
 
 	lim.logical_block_size = blksize;
 	lim.physical_block_size = blksize;
@@ -432,6 +434,8 @@ static u32 req_to_nbd_cmd_type(struct request *req)
 		return NBD_CMD_WRITE;
 	case REQ_OP_READ:
 		return NBD_CMD_READ;
+	case REQ_OP_WRITE_ZEROES:
+		return NBD_CMD_WRITE_ZEROES;
 	default:
 		return U32_MAX;
 	}
@@ -648,6 +652,8 @@ static blk_status_t nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd,
 
 	if (req->cmd_flags & REQ_FUA)
 		nbd_cmd_flags |= NBD_CMD_FLAG_FUA;
+	if ((req->cmd_flags & REQ_NOUNMAP) && (type == NBD_CMD_WRITE_ZEROES))
+		nbd_cmd_flags |= NBD_CMD_FLAG_NO_HOLE;
 
 	/* We did a partial send previously, and we at least sent the whole
 	 * request struct, so just go and send the rest of the pages in the
@@ -1717,6 +1723,8 @@ static int nbd_dbg_flags_show(struct seq_file *s, void *unused)
 		seq_puts(s, "NBD_FLAG_SEND_FUA\n");
 	if (flags & NBD_FLAG_SEND_TRIM)
 		seq_puts(s, "NBD_FLAG_SEND_TRIM\n");
+	if (flags & NBD_FLAG_SEND_WRITE_ZEROES)
+		seq_puts(s, "NBD_FLAG_SEND_WRITE_ZEROES\n");
 
 	return 0;
 }

include/uapi/linux/nbd.h

@@ -42,8 +42,9 @@ enum {
 	NBD_CMD_WRITE = 1,
 	NBD_CMD_DISC = 2,
 	NBD_CMD_FLUSH = 3,
-	NBD_CMD_TRIM = 4
+	NBD_CMD_TRIM = 4,
 	/* userspace defines additional extension commands */
+	NBD_CMD_WRITE_ZEROES = 6,
 };
 
 /* values for flags field, these are server interaction specific. */
@@ -53,11 +54,13 @@ enum {
 #define NBD_FLAG_SEND_FUA	(1 << 3) /* send FUA (forced unit access) */
 #define NBD_FLAG_ROTATIONAL	(1 << 4) /* device is rotational */
 #define NBD_FLAG_SEND_TRIM	(1 << 5) /* send trim/discard */
+#define NBD_FLAG_SEND_WRITE_ZEROES (1 << 6) /* supports WRITE_ZEROES */
 /* there is a gap here to match userspace */
 #define NBD_FLAG_CAN_MULTI_CONN	(1 << 8)	/* Server supports multiple connections per export. */
 
 /* values for cmd flags in the upper 16 bits of request type */
 #define NBD_CMD_FLAG_FUA	(1 << 16) /* FUA (forced unit access) op */
+#define NBD_CMD_FLAG_NO_HOLE	(1 << 17) /* Do not punch a hole for WRITE_ZEROES */
 
 /* These are client behavior specific flags. */
 #define NBD_CFLAG_DESTROY_ON_DISCONNECT	(1 << 0) /* delete the nbd device on