Re: [Devel] [PATCH VZ10 4/6] drivers/md/dm-qcow2: fix seek skip to the next L2 table

[Pavel Tikhomirov <ptikhomirov@virtuozzo.com>]

On 8/10/26 14:29, Andrey Zhadchenko wrote:
> get_next_l2() adds the number of remaining clusters to the current
> sector, so for a position which is not cluster aligned (e.g. the
> llseek offset itself) the result overshoots the L2 table boundary by
> the in-cluster offset.
> 
> Compute the boundary from the cluster index instead of adding to the
> current position.
> 
> https://virtuozzo.atlassian.net/browse/VSTOR-139407
> Feature: dm-qcow2: block device over QCOW2 files driver
> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
> ---
>  drivers/md/dm-qcow2-map.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/md/dm-qcow2-map.c b/drivers/md/dm-qcow2-map.c
> index d6d15b04215c6..776e89fc4eb1b 100644
> --- a/drivers/md/dm-qcow2-map.c
> +++ b/drivers/md/dm-qcow2-map.c
> @@ -4405,12 +4405,12 @@ static struct qio *free_seek_qio_ret_higher(struct qio *qio)
>  static inline sector_t get_next_l2(struct qio *qio)
>  {
>  	struct qcow2 *qcow2 = qio->qcow2;
> -	loff_t start, add;
> +	u64 base_l2, add;
>  
> -	start = to_bytes(qio->bi_iter.bi_sector);
> -	add = qcow2->l2_entries - (start / qcow2->clu_size) % qcow2->l2_entries;
> +	base_l2 = to_bytes(qio->bi_iter.bi_sector) / qcow2->clu_size;
> +	add = qcow2->l2_entries - base_l2 % qcow2->l2_entries;
>  
> -	return qio->bi_iter.bi_sector + (qcow2->clu_size / to_bytes(1)) * add;
> +	return to_sector((base_l2 + add) * qcow2->clu_size);

The above code seems too complex to what it actually tries to accomplish,
it's just an alignment, what about:

static inline sector_t get_next_l2(struct qio *qio)
{
      struct qcow2 *qcow2 = qio->qcow2;
      u64 l2_covered = (u64)qcow2->clu_size * qcow2->l2_entries;

      return to_sector(round_up(to_bytes(qio->bi_iter.bi_sector) + 1,
                                l2_covered));
}

>  }
>  
>  static inline sector_t get_next_clu(struct qio *qio)

-- 
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.