Re: [PATCH 2/2] scsi: scsi_debug: Validate zone access for WRITE ATOMIC (16)

[John Garry <john.garry@linux.dev>]

On 9/17/26 09:45, Niklas Cassel wrote:

So far we have not considered atomic writes for zoned devices - do 
devices which support both technologies exist? Or is this just hypothetical?

> WRITE ATOMIC (16) is a write command, so on a zoned device it is subject
> to the access requirements of the zone that it addresses, and it advances
> the write pointer of a sequential write required zone. See ZBC-3 r06
> (T10/BSR INCITS 579), 4.5.3.3.2 Write access pattern requirements for
> sequential write required zones.
> 
> resp_atomic_write() calls do_device_access() directly, without calling
> check_device_access_params() first and without advancing the write
> pointer afterwards. It is the only command that writes user data which
> does not; WRITE, WRITE SCATTERED and WRITE SAME all go through
> check_device_access_params(), which validates zone access for a zoned
> device.
> 
> As a consequence, with zbc=managed atomic_wr=1, a WRITE ATOMIC (16) can
> write anywhere within a sequential write required zone regardless of its
> write pointer and zone condition, into a gap zone, or across a zone
> boundary, and none of it is reflected in the zone state. The write
> pointer is left where it was, so a subsequent REPORT ZONES does not
> describe the data on the medium, and the next write at that write
> pointer overwrites data that was written without error.
> 
> Validate the access and advance the write pointer the way
> resp_write_dt0() does, holding the zone metadata write lock across both,
> since the write pointer has to be read and updated atomically with
> respect to other commands.
> 
> Assisted-by: LLM
> Fixes: 84f3a3c01d70 ("scsi: scsi_debug: Atomic write support")
> Signed-off-by: Niklas Cassel <cassel@kernel.org>
> ---
> Tested with:
> 
>    modprobe scsi_debug zbc=managed sector_size=512 physblk_exp=3 \
>        zone_size_mb=8 dev_size_mb=128 zone_nr_conv=2 atomic_wr=1
> 
> issuing WRITE ATOMIC (16) with sg_raw. Before this patch, an eight block
> atomic write at the write pointer of an empty sequential write required
> zone completes with GOOD status and leaves the write pointer unchanged,
> and one issued past the write pointer is accepted as well. After it, the
> former advances the write pointer by eight blocks and the latter is
> terminated with ILLEGAL REQUEST / UNALIGNED WRITE COMMAND.
> 
> A two block atomic write at the write pointer, which satisfies
> atomic_wr_gran and atomic_wr_align but is smaller than the 4096 byte
> physical block, is now rejected by the check added in the previous
> patch. An atomic write to a conventional zone is unaffected.
> ---
>   drivers/scsi/scsi_debug.c | 13 +++++++++++++++++
>   1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
> --- a/drivers/scsi/scsi_debug.c
> +++ b/drivers/scsi/scsi_debug.c
> @@ -6043,7 +6043,24 @@
>   		}
>   	}
>   
> +	if (sdebug_dev_is_zoned(devip))
> +		sdeb_meta_write_lock(sip);
> +
> +	ret = check_device_access_params(scp, lba, len, true);
> +	if (ret) {
> +		if (sdebug_dev_is_zoned(devip))
> +			sdeb_meta_write_unlock(sip);
> +		return ret;
> +	}
> +
>   	ret = do_device_access(sip, scp, 0, lba, len, 0, true, true);
> +
> +	/* If ZBC zone then bump its write pointer */
> +	if (sdebug_dev_is_zoned(devip)) {
> +		zbc_inc_wp(devip, lba, len);
> +		sdeb_meta_write_unlock(sip);
> +	}
> +
>   	if (unlikely(ret == -1))
>   		return DID_ERROR << 16;
>   	if (unlikely(ret != len * sdebug_sector_size))