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

[Andrey Zhadchenko]

On 8/14/26 13:09, Pavel Tikhomirov wrote:
> 
> 
> 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 at 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.

seek_qio_set_sector(old_qio, to_sector(end)) will move old qio to the 
next cluster (end) we want to scan in the top image. I don't see any 
problem here. bi_size is rather irrelevant with new code using the end. 
It is only needed to check that calc_front_qio_bytes() return doesn't 
indicate that hole/data starts mid-cluster.
Can you provide a hypothetical example for such infinite loop?


> 
>> -	} 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;
>