Re: [Devel] [PATCH VZ10 5/6] drivers/md/dm-qcow2: scan lower delta in one pass over empty L1 entry

[Pavel Tikhomirov <ptikhomirov@virtuozzo.com>]

On 8/10/26 14:29, Andrey Zhadchenko wrote:
> When an L1 entry is absent and there is a backing image, the seek
> code descends into the lower delta one cluster at a time.
> 
> Change advance_and_spawn_lower_seek_qio() to take limit instead of
> a size. Pass bigger range if L1 entry is empty.
> 
> On a 16G image (128K clusters, extended L2) with no L1 entries over a
> backing image with data at 15G, warm-cache SEEK_DATA improves from
> 40.2 ms to 0.01 ms.
> 
> 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 | 29 +++++++++++++++--------------
>  1 file changed, 15 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/md/dm-qcow2-map.c b/drivers/md/dm-qcow2-map.c
> index 776e89fc4eb1b..6a73c32689eca 100644
> --- a/drivers/md/dm-qcow2-map.c
> +++ b/drivers/md/dm-qcow2-map.c
> @@ -4441,26 +4441,19 @@ static inline void seek_qio_next_clu(struct qio *qio, struct qcow2_map *map)
>  	seek_qio_set_sector(qio, bi_sector);
>  }
>  
> -static struct qio *advance_and_spawn_lower_seek_qio(struct qio *old_qio, u32 size)
> +static struct qio *advance_and_spawn_lower_seek_qio(struct qio *old_qio, loff_t end)
>  {
> +	struct qcow2 *lower = old_qio->qcow2->lower;
>  	struct qio *new_qio;
> -	loff_t start;
>  
> -	start = to_bytes(old_qio->bi_iter.bi_sector);
> +	if (end > lower->hdr.size)
> +		end = lower->hdr.size;
>  
> -	if (start + size > old_qio->qcow2->lower->hdr.size)
> -		size = old_qio->qcow2->lower->hdr.size - start;
> -
> -	new_qio = alloc_seek_qio(old_qio->qcow2->lower, old_qio, start + size);
> +	new_qio = alloc_seek_qio(lower, old_qio, end);
>  	if (!new_qio)
>  		return NULL;
>  
> -	if (old_qio->bi_iter.bi_size == size) {
> -		seek_qio_next_clu(old_qio, NULL);

Removal of this branch is a bit suspicious. Previously we advanced old_qio to next cluster
after bi_size should become zero. After this change we don't advance. AI suggests that
this can lead to infinite loop as then we will return back from new_qio to old_qio it won't
continue the search correctly.

> -	} else {
> -		old_qio->bi_iter.bi_sector += to_sector(size);
> -		old_qio->bi_iter.bi_size -= size;
> -	}
> +	seek_qio_set_sector(old_qio, to_sector(end));
>  
>  	return new_qio;
>  }
> @@ -4508,8 +4501,16 @@ static int qcow2_llseek_hole_qio(struct qio *qio, int whence, loff_t *result)
>  			 */
>  			if (to_bytes(qio->bi_iter.bi_sector) < qio->qcow2->lower->hdr.size) {
>  				struct qio *new_qio;
> +				loff_t end;
> +
> +				if (!(map.level & L2_LEVEL))
> +					end = min_t(loff_t,
> +						    to_bytes(get_next_l2(qio)),
> +						    SEEK_QIO_DATA(qio)->lim);
> +				else
> +					end = to_bytes(qio->bi_iter.bi_sector) + size;
>  
> -				new_qio = advance_and_spawn_lower_seek_qio(qio, size);
> +				new_qio = advance_and_spawn_lower_seek_qio(qio, end);
>  				if (!new_qio) {
>  					ret = -ENOMEM;
>  					break;

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