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

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Date:   Fri, 23 Oct 2020 10:13:46 +0100
From:   "hch@...radead.org" <hch@...radead.org>
To:     Mike Snitzer <snitzer@...hat.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Sergei Shtepa <sergei.shtepa@...am.com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Hannes Reinecke <hare@...e.de>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "hch@...radead.org" <hch@...radead.org>,
        "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

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);