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Date: Sun, 2 Oct 2011 16:46:58 +0400
From: Lev Serebryakov <lev@FreeBSD.org>
Organization: FreeBSD
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Subject: What is geom_start() and struct bio contract?
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Hello, Freebsd-geom.

 What is contract of struct bio passed to geom_start()? Why, for
example, geom_nop (and all others) does clone like this:

        cbp =3D g_clone_bio(bp);
        if (cbp =3D=3D NULL) {
                g_io_deliver(bp, ENOMEM);
                return;
        }
        cbp->bio_done =3D g_std_done;
        cbp->bio_offset =3D bp->bio_offset + sc->sc_offset;
        cbp->bio_data =3D bp->bio_data;
        cbp->bio_length =3D bp->bio_length;
        pp =3D LIST_FIRST(&gp->provider);
        KASSERT(pp !=3D NULL, ("NULL pp"));
        cbp->bio_to =3D pp;
        G_NOP_LOGREQ(cbp, "Sending request.");
        g_io_request(cbp, LIST_FIRST(&gp->consumer));

 And doesn't modify offset and other fields of original bio?

 And second question about this code. Here are lines:

        pp =3D LIST_FIRST(&gp->provider);
        KASSERT(pp !=3D NULL, ("NULL pp"));
        cbp->bio_to =3D pp;

 Which put pointer to NOP's provider to bio_to field. But later,
 g_io_request() reset this field to provider, which is aatached to
 target consumer:

         pp =3D cp->provider;
         ...
         bp->bio_to =3D pp;

 Is it Ok? What should store this field? If it should store provider,
 which ISSUED this bio, why g_io_request() reset it? If it should
 store provider, which will EXECUTE this bio, why geom_nop set it to
 itself provider, and why is here this field at all, as it could be
 derived from bio_from (it seems, that it is always set to
 bio_from->provider!)?

--=20
// Black Lion AKA Lev Serebryakov <lev@FreeBSD.org>