On Thu, Jul 07, 2022 at 11:16:42AM +0200, Sergei Shtepa wrote: > The module creates a block device for each snapshot image. > To make a backup of a block device, the backup tool reads snapshot image. > This snapshot image block device allows to mount a file system on it > and perform the necessary preparation. If not for this requirement, > it would be possible to implement reading via an additional ioctl. > But that wouldn't be a good design, I think. Ok, got it. It was just me who was confused. > > Perhaps I have implemented this block device incorrectly? > Processing requests of the snapshot image block device is started > in the function snapimage_queue_rq(). And ends in the > snapimage_queue_work() in another kernel thread. Therefore, when > the request is initialized in snapimage_init_request(), a kernel worker > is prepared. I don't think it is wrong, but there is some potential for optimization. Is there a reson this is implemented as a blk-mq driver vs a bio based driver that just implements ->submit_bio? The latter has the advantage that you are always called in user context, and don't need the extra workqueue offload. The block layer also generally assumes that blk-mq drivers don't call submit_bio_noacct underneath, so if we can't I'd much prefer the bio based driver approach here.
