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

[Sergei Shtepa]

The 10/23/2020 13:31, Hannes Reinecke wrote:
> On 10/23/20 11:13 AM, hch@xxxxxxxxxxxxx wrote:
> > On Thu, Oct 22, 2020 at 01:54:16PM -0400, Mike Snitzer wrote:
> >> On Thu, Oct 22, 2020 at 11:14 AM Darrick J. Wong
> >>> Stupid question: Why don't you change the block layer to make it
> >>> possible to insert device mapper devices after the blockdev has been set
> >>> up?
> >>
> >> Not a stupid question.  Definitely something that us DM developers
> >> have wanted to do for a while.  Devil is in the details but it is the
> >> right way forward.
> >>
> > 
> > Yes, I think that is the right thing to do.  And I don't think it should
> > be all that hard.  All we'd need in the I/O path is something like the
> > pseudo-patch below, which will allow the interposer driver to resubmit
> > bios using submit_bio_noacct as long as the driver sets BIO_INTERPOSED.
> > 
> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index ac00d2fa4eb48d..3f6f1eb565e0a8 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -1051,6 +1051,9 @@ blk_qc_t submit_bio_noacct(struct bio *bio)
> >   		return BLK_QC_T_NONE;
> >   	}
> >   
> > +	if (blk_has_interposer(bio->bi_disk) &&
> > +	    !(bio->bi_flags & BIO_INTERPOSED))
> > +		return __submit_bio_interposed(bio);
> >   	if (!bio->bi_disk->fops->submit_bio)
> >   		return __submit_bio_noacct_mq(bio);
> >   	return __submit_bio_noacct(bio);
> > 

It`s will be great! Approximately this interception capability is not
enough now.

> My thoughts went more into the direction of hooking into ->submit_bio, 
> seeing that it's a NULL pointer for most (all?) block drivers.
> 
> But sure, I'll check how the interposer approach would turn out.

If anyone will do the patch blk-interposer, please add me to CC.
I will try to offer my module that will use blk-interposer.

-- 
Sergei Shtepa
Veeam Software developer.