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

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Date:   Fri, 23 Oct 2020 14:04:07 +0300
From:   Sergei Shtepa <sergei.shtepa@...am.com>
To:     Hannes Reinecke <hare@...e.de>
CC:     "hch@...radead.org" <hch@...radead.org>,
        Mike Snitzer <snitzer@...hat.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        "Damien Le Moal" <Damien.LeMoal@....com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "rjw@...ysocki.net" <rjw@...ysocki.net>,
        "len.brown@...el.com" <len.brown@...el.com>,
        "pavel@....cz" <pavel@....cz>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        Johannes Thumshirn <Johannes.Thumshirn@....com>,
        "ming.lei@...hat.com" <ming.lei@...hat.com>,
        "jack@...e.cz" <jack@...e.cz>, "tj@...nel.org" <tj@...nel.org>,
        "gustavo@...eddedor.com" <gustavo@...eddedor.com>,
        "bvanassche@....org" <bvanassche@....org>,
        "osandov@...com" <osandov@...com>,
        "koct9i@...il.com" <koct9i@...il.com>,
        "steve@....org" <steve@....org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        device-mapper development <dm-devel@...hat.com>,
        Alasdair G Kergon <agk@...hat.com>
Subject: Re: [PATCH 0/2] block layer filter and block device snapshot module

The 10/23/2020 13:31, Hannes Reinecke wrote:
> On 10/23/20 11:13 AM, hch@...radead.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.