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    SubjectRe: [PATCH 0/2] block layer filter and block device snapshot module
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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
    SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
    HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

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