Re: [PATCH 0/2] block layer filter and block device snapshot module

[Mike Snitzer]

On Wed, Oct 21, 2020 at 5:04 AM Sergei Shtepa <sergei.shtepa@xxxxxxxxx> wrote:
>
> Hello everyone! Requesting for your comments and suggestions.
>
> # blk-filter
>
> Block layer filter allows to intercept BIO requests to a block device.
>
> Interception is performed at the very beginning of the BIO request
> processing, and therefore does not affect the operation of the request
> processing queue. This also makes it possible to intercept requests from
> a specific block device, rather than from the entire disk.
>
> The logic of the submit_bio function has been changed - since the
> function execution results are not processed anywhere (except for swap
> and direct-io) the function won't return a value anymore.

Your desire to switch to a void return comes exactly when I've noticed
we need it.

->submit_bio's blk_qc_t return is the cookie assigned by blk-mq.  Up
to this point we haven't actually used it for bio-based devices but it
seems clear we'll soon need for bio-based IO polling support.

Just today, I've been auditing drivers/md/dm.c with an eye toward
properly handling the blk_qc_t return (or lack thereof) from various
DM methods.

It could easily be that __submit_bio_noacct and __submit_bio_noacct_mq
will be updated to do something meaningful with the returned cookie
(or that DM will) to facilitate proper IO polling.

Mike