[dm-devel] [PATCH 0/2] block layer filter and block device snapshot module
Hannes Reinecke
hare at suse.de
Fri Oct 23 10:31:05 UTC 2020
On 10/23/20 11:13 AM, hch at 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 at suse.de +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer
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