Re: [PATCH v3 5/5] dm: Check for forbidden splitting of zone write operations

[Mikulas Patocka <mpatocka@redhat.com>]

On Wed, 25 Jun 2025, Damien Le Moal wrote:

> DM targets must not split zone append and write operations using
> dm_accept_partial_bio() as doing so is forbidden for zone append BIOs,
> breaks zone append emulation using regular write BIOs and potentially
> creates deadlock situations with queue freeze operations.
> 
> Modify dm_accept_partial_bio() to add missing BUG_ON() checks for all
> these cases, that is, check that the BIO is a write or write zeroes
> operation. This change packs all the zone related checks together under
> a static_branch_unlikely(&zoned_enabled) and done only if the target is
> a zoned device.
> 
> Fixes: f211268ed1f9 ("dm: Use the block layer zone append emulation")
> Cc: stable@vger.kernel.org
> Signed-off-by: Damien Le Moal <dlemoal@kernel.org>

Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>

> ---
>  drivers/md/dm.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index f1e63c1808b4..f82457e7eed1 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -1286,8 +1286,9 @@ static size_t dm_dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
>  /*
>   * A target may call dm_accept_partial_bio only from the map routine.  It is
>   * allowed for all bio types except REQ_PREFLUSH, REQ_OP_ZONE_* zone management
> - * operations, REQ_OP_ZONE_APPEND (zone append writes) and any bio serviced by
> - * __send_duplicate_bios().
> + * operations, zone append writes (native with REQ_OP_ZONE_APPEND or emulated
> + * with write BIOs flagged with BIO_EMULATES_ZONE_APPEND) and any bio serviced
> + * by __send_duplicate_bios().
>   *
>   * dm_accept_partial_bio informs the dm that the target only wants to process
>   * additional n_sectors sectors of the bio and the rest of the data should be
> @@ -1320,11 +1321,19 @@ void dm_accept_partial_bio(struct bio *bio, unsigned int n_sectors)
>  	unsigned int bio_sectors = bio_sectors(bio);
>  
>  	BUG_ON(dm_tio_flagged(tio, DM_TIO_IS_DUPLICATE_BIO));
> -	BUG_ON(op_is_zone_mgmt(bio_op(bio)));
> -	BUG_ON(bio_op(bio) == REQ_OP_ZONE_APPEND);
>  	BUG_ON(bio_sectors > *tio->len_ptr);
>  	BUG_ON(n_sectors > bio_sectors);
>  
> +	if (static_branch_unlikely(&zoned_enabled) &&
> +	    unlikely(bdev_is_zoned(bio->bi_bdev))) {
> +		enum req_op op = bio_op(bio);
> +
> +		BUG_ON(op_is_zone_mgmt(op));
> +		BUG_ON(op == REQ_OP_WRITE);
> +		BUG_ON(op == REQ_OP_WRITE_ZEROES);
> +		BUG_ON(op == REQ_OP_ZONE_APPEND);
> +	}
> +
>  	*tio->len_ptr -= bio_sectors - n_sectors;
>  	bio->bi_iter.bi_size = n_sectors << SECTOR_SHIFT;
>  
> -- 
> 2.49.0
>