[Devel] [PATCH rh9 v2] dm-zero-req: Introduce zero request based target

[Kirill Tkhai]

On 05.04.2022 16:52, Konstantin Khorenko wrote:
> This driver is like "dm-zero", but request based rather than bio based
> like original "dm-zero".
> 
> This driver will be used on a block device for Container configuration
> stage: we need to construct a block device which honors CBT mask (stored
> in ploop image), for that first we need to create a dummy block device
> (pure technical issue, otherwise CBT mask is dropped).
> 
> dm-ploop/dm-qcow2 are request based, thus we need zero target
> also to be request based.
> 
> https://jira.sw.ru/browse/PSBM-134130
> 
> Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
> Feature: cbt: changed block tracking (for backup)

Reviewed-by: Kirill Tkhai <kirill.tkhai at openvz.org>
 
> ---
> v2: * dropped extra kernel config option, put new modules under CONFIG_DM_ZERO
>     * dropped "readahead of null bytes" optimization
> ---
>  drivers/md/Makefile      |  1 +
>  drivers/md/dm-zero-req.c | 91 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 92 insertions(+)
>  create mode 100644 drivers/md/dm-zero-req.c
> 
> diff --git a/drivers/md/Makefile b/drivers/md/Makefile
> index 94134440cf70..3197d24c0e75 100644
> --- a/drivers/md/Makefile
> +++ b/drivers/md/Makefile
> @@ -76,6 +76,7 @@ obj-$(CONFIG_DM_PERSISTENT_DATA) += persistent-data/
>  obj-$(CONFIG_DM_MIRROR)		+= dm-mirror.o dm-log.o dm-region-hash.o
>  obj-$(CONFIG_DM_LOG_USERSPACE)	+= dm-log-userspace.o
>  obj-$(CONFIG_DM_ZERO)		+= dm-zero.o
> +obj-$(CONFIG_DM_ZERO)		+= dm-zero-req.o
>  obj-$(CONFIG_DM_RAID)		+= dm-raid.o
>  obj-$(CONFIG_DM_THIN_PROVISIONING) += dm-thin-pool.o
>  obj-$(CONFIG_DM_VERITY)		+= dm-verity.o
> diff --git a/drivers/md/dm-zero-req.c b/drivers/md/dm-zero-req.c
> new file mode 100644
> index 000000000000..9e44de15dcd6
> --- /dev/null
> +++ b/drivers/md/dm-zero-req.c
> @@ -0,0 +1,91 @@
> +/*
> + * Copyright (C) 2003 Jana Saout <jana at saout.de>
> + *
> + * This file is released under the GPL.
> + */
> +
> +#include <linux/device-mapper.h>
> +
> +#include <linux/module.h>
> +#include <linux/init.h>
> +#include <linux/bio.h>
> +#include <linux/blk-mq.h>
> +#include "dm-rq.h"
> +
> +#define DM_MSG_PREFIX "zero"
> +
> +/*
> + * Construct a dummy mapping that only returns zeros
> + */
> +static int zero_ctr(struct dm_target *ti, unsigned int argc, char **argv)
> +{
> +	if (argc != 0) {
> +		ti->error = "No arguments required";
> +		return -EINVAL;
> +	}
> +
> +	/*
> +	 * Silently drop discards, avoiding -EOPNOTSUPP.
> +	 */
> +	ti->num_discard_bios = 1;
> +
> +	return 0;
> +}
> +
> +static int zero_clone_and_map_rq(struct dm_target *ti, struct request *rq,
> +				 union map_info *map_context,
> +				 struct request **clone)
> +{
> +	struct bio *bio = rq->bio;
> +
> +	switch (bio_op(bio)) {
> +	case REQ_OP_READ:
> +		while (bio) {
> +			zero_fill_bio(bio);
> +			bio = bio->bi_next;
> +		}
> +
> +		break;
> +	case REQ_OP_WRITE:
> +		/* writes get silently dropped */
> +		break;
> +	default:
> +		return DM_MAPIO_KILL;
> +	}
> +
> +	dm_complete_request(rq, BLK_STS_OK);
> +
> +	/* accepted rq, don't make new request */
> +	return DM_MAPIO_SUBMITTED;
> +}
> +
> +static struct target_type zero_target = {
> +	.name   = "zero-rq",
> +	.version = {1, 1, 0},
> +	.features = DM_TARGET_NOWAIT,
> +	.module = THIS_MODULE,
> +	.ctr    = zero_ctr,
> +	.clone_and_map_rq = zero_clone_and_map_rq,
> +};
> +
> +static int __init dm_zero_init(void)
> +{
> +	int r = dm_register_target(&zero_target);
> +
> +	if (r < 0)
> +		DMERR("register failed %d", r);
> +
> +	return r;
> +}
> +
> +static void __exit dm_zero_exit(void)
> +{
> +	dm_unregister_target(&zero_target);
> +}
> +
> +module_init(dm_zero_init)
> +module_exit(dm_zero_exit)
> +
> +MODULE_AUTHOR("Jana Saout <jana at saout.de>");
> +MODULE_DESCRIPTION(DM_NAME " dummy request based target returning zeros");
> +MODULE_LICENSE("GPL");