Re: [PATCH 15/20] block, blksnap: snapshot image block device

From: Sergei Shtepa
Date: Fri Jul 08 2022 - 03:58:42 EST

> 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.

There was a goal to use the kernel innovations whenever possible.
Of course, it makes sense to return to bio based if it allows to
achieve better performance. This is not a matter of principle.