Re: [PATCH 1/2] ublk: support ublk recovery

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Hi Chengyu,

On 2026/6/19 12:19, Chengyu wrote:
> From: Chengyu Zhu <hudsonzhu@tencent.com>
> 
> Enable ublk user recovery so erofsmount can reattach a daemon to a
> recoverable ublk device after the previous userspace server exits.
> 
> Store the recovery source information in a ublk-specific runtime record,
> create ublk devices with user recovery enabled, and add the mount-side
> reattach path for existing recoverable ublk devices.
> 
> Also encode ublk IO uring command opcodes explicitly for non-legacy kernels,
> and initialize the control ring before querying whether a ublk device is
> recoverable.
> 
> Signed-off-by: Chengyu Zhu <hudsonzhu@tencent.com>
> ---
>   lib/backends/ublk.c |  23 ++++++---
>   mount/main.c        | 115 ++++++++++++++++++++++++++++++++++++++++----
>   2 files changed, 122 insertions(+), 16 deletions(-)
> 
> diff --git a/lib/backends/ublk.c b/lib/backends/ublk.c
> index a8ecad9..49191de 100644
> --- a/lib/backends/ublk.c
> +++ b/lib/backends/ublk.c
> @@ -258,7 +258,16 @@ static unsigned int erofsublk_formalize_cmd_op(unsigned int op)
>   	DBG_BUGON(_IOC_DIR(op) != 0);
>   	DBG_BUGON(_IOC_SIZE(op) != 0);
>   
> -	if (op < ARRAY_SIZE(ctrl_cmd_op) && !erofs_ublk_use_legacy_cmds)
> +	if (erofs_ublk_use_legacy_cmds)
> +		return op;
> +
> +	/* IO opcodes live above the ctrl table and need explicit encoding */
> +	if (op == UBLK_IO_FETCH_REQ)
> +		return UBLK_U_IO_FETCH_REQ;
> +	if (op == UBLK_IO_COMMIT_AND_FETCH_REQ)
> +		return UBLK_U_IO_COMMIT_AND_FETCH_REQ;
> +
> +	if (op < ARRAY_SIZE(ctrl_cmd_op))
>   		return ctrl_cmd_op[op];
>   	return op;

	if (!erofs_ublk_use_legacy_cmds) {
		if (op == UBLK_IO_FETCH_REQ)
			return UBLK_U_IO_FETCH_REQ;
		if (op == UBLK_IO_COMMIT_AND_FETCH_REQ)
			return UBLK_U_IO_COMMIT_AND_FETCH_REQ;
		if (op < ARRAY_SIZE(ctrl_cmd_op))
			return ctrl_cmd_op[op];
	}
	return op;

?


>   }
> @@ -528,11 +537,6 @@ static inline unsigned int user_data_to_tag(u64 user_data)
>   	return user_data & 0xffff;
>   }

..

>   
> +static int ublk_dev_id_from_path(const char *path);

Can we avoid forward declaration?

> +
> +static int erofsmount_ublk_reattach(int dev_id)
> +{
> +	struct erofsmount_nbd_ctx ctx = { .vd = &ctx._vd };
> +	char *recp;
> +	FILE *f;
> +	int err;
> +
> +	if (!erofs_ublk_is_recoverable(dev_id))
> +		return -EINVAL;
> +
> +	if (asprintf(&recp, EROFSMOUNT_UBLK_REC_FMT, dev_id) <= 0)
> +		return -ENOMEM;
> +
> +	f = fopen(recp, "r");
> +	if (!f) {
> +		err = -errno;
> +		free(recp);
> +		return err;
> +	}
> +
> +	err = erofsmount_open_recovery_source(&ctx, f);
> +	if (err) {
> +		free(recp);
> +		return err;
> +	}
> +
> +	if (fork() == 0) {
> +		err = erofs_ublk_recover_dev(dev_id, erofsmount_ublk_handler,
> +					     ctx.vd);
> +		if (err) {
> +			erofs_err("ublk recover dev %d failed: %s",
> +				  dev_id, strerror(-err));
> +			erofs_io_close(ctx.vd);
> +			exit(EXIT_FAILURE);
> +		}
> +		err = erofs_ublk_start(dev_id, -1);
> +		erofs_ublk_destroy(dev_id);
> +		erofs_io_close(ctx.vd);
> +		(void)unlink(recp);
> +		exit(err ? EXIT_FAILURE : EXIT_SUCCESS);
> +	}
> +
> +	erofs_io_close(ctx.vd);
> +	free(recp);
> +	return 0;
> +}
> +
>   static int erofsmount_reattach(const char *target)
>   {
>   	struct erofsmount_nbd_ctx ctx = { .vd = &ctx._vd };
> @@ -1450,6 +1528,10 @@ static int erofsmount_reattach(const char *target)
>   	if (err < 0)
>   		return -errno;
>   
> +	nbdnum = ublk_dev_id_from_path(target);
> +	if (S_ISBLK(st.st_mode) && nbdnum >= 0)
> +		return erofsmount_ublk_reattach(nbdnum);
> +
>   	if (!S_ISBLK(st.st_mode) || major(st.st_rdev) != EROFS_NBD_MAJOR)
>   		return -ENOTBLK;

	if (!S_ISBLK(st.st_mode))
		return -ENOTBLK;

	nbdnum = ublk_dev_id_from_path(target);
	if (nbdnum >= 0)
		return erofsmount_ublk_reattach(nbdnum);

	if (major(st.st_rdev) != EROFS_NBD_MAJOR)
		return -ENOTBLK;

?


Thanks,
Gao Xiang