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Date:   Fri, 23 Oct 2020 14:04:07 +0300
From:   Sergei Shtepa <sergei.shtepa@veeam.com>
To:     Hannes Reinecke <hare@suse.de>
CC:     "hch@infradead.org" <hch@infradead.org>,
        Mike Snitzer <snitzer@redhat.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
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        Johannes Thumshirn <Johannes.Thumshirn@wdc.com>,
        "ming.lei@redhat.com" <ming.lei@redhat.com>,
        "jack@suse.cz" <jack@suse.cz>, "tj@kernel.org" <tj@kernel.org>,
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        "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        device-mapper development <dm-devel@redhat.com>,
        Alasdair G Kergon <agk@redhat.com>
Subject: Re: [PATCH 0/2] block layer filter and block device snapshot module
Message-ID: <20201023110407.GA23020@veeam.com>
References: <71926887-5707-04a5-78a2-ffa2ee32bd68@suse.de>
 <20201021141044.GF20749@veeam.com>
 <ca8eaa40-b422-2272-1fd9-1d0a354c42bf@suse.de>
 <20201022094402.GA21466@veeam.com>
 <BL0PR04MB6514AC1B1FF313E6A14D122CE71D0@BL0PR04MB6514.namprd04.prod.outlook.com>
 <20201022135213.GB21466@veeam.com>
 <20201022151418.GR9832@magnolia>
 <CAMM=eLfO_L-ZzcGmpPpHroznnSOq_KEWignFoM09h7Am9yE83g@mail.gmail.com>
 <20201023091346.GA25115@infradead.org>
 <d50062cd-929d-c8ff-5851-4e1d517dc4cb@suse.de>
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The 10/23/2020 13:31, Hannes Reinecke wrote:
> On 10/23/20 11:13 AM, hch@infradead.org 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.