Re: [RFC v2 00/11] dm-pcache – persistent-memory cache for block devices

[Dongsheng Yang <dongsheng.yang@linux.dev>]

在 6/30/2025 11:57 PM, Mikulas Patocka 写道:
>
> On Mon, 23 Jun 2025, Dongsheng Yang wrote:
>
>> +static int dm_pcache_map_bio(struct dm_target *ti, struct bio *bio)
>> +{
>> +     struct pcache_request *pcache_req = dm_per_bio_data(bio, sizeof(struct pcache_request));
>> +     struct dm_pcache *pcache = ti->private;
>> +     int ret;
>> +
>> +     pcache_req->pcache = pcache;
>> +     kref_init(&pcache_req->ref);
>> +     pcache_req->ret = 0;
>> +     pcache_req->bio = bio;
>> +     pcache_req->off = (u64)bio->bi_iter.bi_sector << SECTOR_SHIFT;
>> +     pcache_req->data_len = bio->bi_iter.bi_size;
>> +     INIT_LIST_HEAD(&pcache_req->list_node);
>> +     bio->bi_iter.bi_sector = dm_target_offset(ti, bio->bi_iter.bi_sector);
>>
>> This looks suspicious because you store the original bi_sector to
>> pcache_req->off and then subtract the target offset from it. Shouldn't
>> "bio->bi_iter.bi_sector = dm_target_offset(ti, bio->bi_iter.bi_sector);"
>> be before "pcache_req->off = (u64)bio->bi_iter.bi_sector <<
>> SECTOR_SHIFT;"?
>>
>>
>> Yes, that logic is indeed questionable, but it works in testing.
>>
>> Since we define dm-pcache as a **singleton**, both behaviors should
>> effectively be equivalent, IIUC. Also, in V1 I moved the call to
>> `dm_target_offset()` so it runs before setting up `pcache_req->off`,
>> making the code logic correct.
> If this target is singleton, you can delete the call to dm_target_offset
> at all.
>
> That call is harmless, but it looks confusing when reviewing the code,
> because pcache_req->off is set to the absolute bio sector (from the start
> of the table) and bio->bi_iter.bi_sector is set to the relative bio sector
> (from the start of the target). If the target always starts at offset 0,
> dm_target_offset just returns bi_sector.


That makes sense

Thanx

>
> Mikulas
>