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Subject: Re: [PATCH 0/2] block layer filter and block device snapshot module
To:     "hch@infradead.org" <hch@infradead.org>,
        Mike Snitzer <snitzer@redhat.com>
Cc:     "Darrick J. Wong" <darrick.wong@oracle.com>,
        Sergei Shtepa <sergei.shtepa@veeam.com>,
        Damien Le Moal <Damien.LeMoal@wdc.com>,
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        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        Johannes Thumshirn <Johannes.Thumshirn@wdc.com>,
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        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        device-mapper development <dm-devel@redhat.com>,
        Alasdair G Kergon <agk@redhat.com>
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From:   Hannes Reinecke <hare@suse.de>
Message-ID: <d50062cd-929d-c8ff-5851-4e1d517dc4cb@suse.de>
Date:   Fri, 23 Oct 2020 12:31:05 +0200
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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);
> 
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.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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