[Devel] [PATCH VZ10 2/6] drivers/md/dm-qcow2: keep seek parse window within limit

[Andrey Zhadchenko]

On 8/14/26 11:55, Pavel Tikhomirov wrote:
> 
> 
> On 8/10/26 14:29, Andrey Zhadchenko wrote:
>> seek_qio_next_clu() always sets the window size to the whole
>> cluster, so the window may cross the scan limit. The top level qio
>> then parses metadata past the virtual disk end if the disk size is
>> not cluster aligned, and SEEK_HOLE may report "sector + size" past
>> the limit.
>> This is especially painful for lower delta seeks, since at top
>> level this is luckily alleviated by blkdev_llseek_wrapper().
>>
>> Add seek_qio_set_sector(), which sets bi_sector and also clamps
>> bi_size if needed.
>>
>> 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, 17 insertions(+), 12 deletions(-)
>>
>> diff --git a/drivers/md/dm-qcow2-map.c b/drivers/md/dm-qcow2-map.c
>> index 542c7949321e5..342e4af688509 100644
>> --- a/drivers/md/dm-qcow2-map.c
>> +++ b/drivers/md/dm-qcow2-map.c
>> @@ -4352,6 +4352,16 @@ struct qio_data_llseek_hole {
>>   
>>   #define SEEK_QIO_DATA(qio) ((struct qio_data_llseek_hole *)qio->data)
>>   
>> +static void seek_qio_set_sector(struct qio *qio, sector_t bi_sector)
>> +{
>> +	loff_t pos = to_bytes(bi_sector);
>> +	loff_t end = round_up(pos + 1, qio->qcow2->clu_size);
>> +
>> +	qio->bi_iter.bi_sector = bi_sector;
>> +	end = min_t(loff_t, end, SEEK_QIO_DATA(qio)->lim);
>> +	qio->bi_iter.bi_size = end > pos ? end - pos : 0;
>> +}
>> +
>>   static struct qio *alloc_seek_qio(struct qcow2 *qcow2, struct qio *parent, loff_t new_lim)
>>   {
>>   	struct qio_data_llseek_hole *data;
>> @@ -4376,12 +4386,7 @@ static struct qio *alloc_seek_qio(struct qcow2 *qcow2, struct qio *parent, loff_
>>   
>>   	if (parent) {
>>   		data->higher = parent;
>> -		qio->bi_iter.bi_sector = parent->bi_iter.bi_sector;
>> -
>> -		if (to_bytes(qio->bi_iter.bi_sector) + parent->bi_iter.bi_size > new_lim)
>> -			qio->bi_iter.bi_size = new_lim - to_bytes(qio->bi_iter.bi_sector);
>> -		else
>> -			qio->bi_iter.bi_size = parent->bi_iter.bi_size;
>> +		seek_qio_set_sector(qio, parent->bi_iter.bi_sector);
> 
> Can new_lim be smaller than the end derived from qio->qcow2->clu_size?
> It looks like now we ignore new_lim completely.

A few lines above we set
	data->lim = new_lim;
seek_qio_set_sector respects this data->lim

> 
>>   	}
>>   
>>   	return qio;
>> @@ -4422,16 +4427,18 @@ static inline sector_t get_next_clu(struct qio *qio)
>>   
>>   static inline void seek_qio_next_clu(struct qio *qio, struct qcow2_map *map)
>>   {
>> +	sector_t bi_sector;
>> +
>>   	/*
>>   	 * Whole L2 table is unmapped - skip to next l2 table,
>>   	 * but only if there is no backing image
>>   	 */
>>   	if (map && !(map->level & L2_LEVEL) && !qio->qcow2->lower)
>> -		qio->bi_iter.bi_sector = get_next_l2(qio);
>> +		bi_sector = get_next_l2(qio);
>>   	else
>> -		qio->bi_iter.bi_sector = get_next_clu(qio);
>> +		bi_sector = get_next_clu(qio);
>>   
>> -	qio->bi_iter.bi_size = qio->qcow2->clu_size;
>> +	seek_qio_set_sector(qio, bi_sector);
>>   }
>>   
>>   static struct qio *advance_and_spawn_lower_seek_qio(struct qio *old_qio, u32 size)
>> @@ -4567,9 +4574,7 @@ loff_t qcow2_llseek_hole(struct dm_target *ti, loff_t offset, int whence)
>>   	if (!qio)
>>   		return -ENOMEM;
>>   
>> -	qio->bi_iter.bi_sector = to_sector(offset);
>> -	qio->bi_iter.bi_size = qcow2->clu_size -
>> -			       to_bytes(qio->bi_iter.bi_sector) % qcow2->clu_size;
>> +	seek_qio_set_sector(qio, to_sector(offset));
>>   
>>   	ret = qcow2_llseek_hole_qio(qio, whence, &result);
>>   	/* In case of error remap ENXIO as it have special meaning for llseek */
>