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.
Mikulas
