Function dbuf_whichblock

freebsd/contrib/openzfs/module/zfs/dbuf.c:1168–1208

* Calculate which level n block references the data at the level 0 offset * provided. */

 * provided.
 */
uint64_t
dbuf_whichblock(const dnode_t *dn, const int64_t level, const uint64_t offset)
{
	if (dn->dn_datablkshift != 0 && dn->dn_indblkshift != 0) {
		/*
		 * The level n blkid is equal to the level 0 blkid divided by
		 * the number of level 0s in a level n block.
		 *
		 * The level 0 blkid is offset >> datablkshift =
		 * offset / 2^datablkshift.
		 *
		 * The number of level 0s in a level n is the number of block
		 * pointers in an indirect block, raised to the power of level.
		 * This is 2^(indblkshift - SPA_BLKPTRSHIFT)^level =
		 * 2^(level*(indblkshift - SPA_BLKPTRSHIFT)).
		 *
		 * Thus, the level n blkid is: offset /
		 * ((2^datablkshift)*(2^(level*(indblkshift-SPA_BLKPTRSHIFT))))
		 * = offset / 2^(datablkshift + level *
		 *   (indblkshift - SPA_BLKPTRSHIFT))
		 * = offset >> (datablkshift + level *
		 *   (indblkshift - SPA_BLKPTRSHIFT))
		 */

		const unsigned exp = dn->dn_datablkshift +
		    level * (dn->dn_indblkshift - SPA_BLKPTRSHIFT);

		if (exp >= 8 * sizeof (offset)) {
			/* This only happens on the highest indirection level */
			ASSERT3U(level, ==, dn->dn_nlevels - 1);
			return (0);
		}

		ASSERT3U(exp, <, 8 * sizeof (offset));

		return (offset >> exp);
	} else {
		ASSERT3U(offset, <, dn->dn_datablksz);
		return (0);
	}
}

/*
 * This function is used to lock the parent of the provided dbuf. This should be