On 10/23/20 11:13 AM, hch@xxxxxxxxxxxxx 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@xxxxxxx +49 911 74053 688
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