+/**
+ * Unpack an OST object id/seq (group) into a FID. This is needed for
+ * converting all obdo, lmm, lsm, etc. 64-bit id/seq pairs into proper
+ * FIDs. Note that if an id/seq is already in FID/IDIF format it will
+ * be passed through unchanged. Only legacy OST objects in "group 0"
+ * will be mapped into the IDIF namespace so that they can fit into the
+ * struct lu_fid fields without loss. For reference see:
+ * http://arch.lustre.org/index.php?title=Interoperability_fids_zfs
+ */
+static inline int ostid_to_fid(struct lu_fid *fid, const struct ost_id *ostid,
+ __u32 ost_idx)
+{
+ obd_seq seq = ostid_seq(ostid);
+
+ if (ost_idx > 0xffff) {
+ CERROR("bad ost_idx, "DOSTID" ost_idx:%u\n", POSTID(ostid),
+ ost_idx);
+ return -EBADF;
+ }
+
+ if (fid_seq_is_mdt0(seq)) {
+ obd_id oid = ostid_id(ostid);
+
+ /* This is a "legacy" (old 1.x/2.early) OST object in "group 0"
+ * that we map into the IDIF namespace. It allows up to 2^48
+ * objects per OST, as this is the object namespace that has
+ * been in production for years. This can handle create rates
+ * of 1M objects/s/OST for 9 years, or combinations thereof. */
+ if (oid >= IDIF_MAX_OID) {
+ CERROR("bad MDT0 id, "DOSTID" ost_idx:%u\n",
+ POSTID(ostid), ost_idx);
+ return -EBADF;
+ }
+ fid->f_seq = fid_idif_seq(oid, ost_idx);
+ /* truncate to 32 bits by assignment */
+ fid->f_oid = oid;
+ /* in theory, not currently used */
+ fid->f_ver = oid >> 48;
+ } else if (likely(!fid_seq_is_default(seq)))
+ /* if (fid_seq_is_idif(seq) || fid_seq_is_norm(seq)) */ {
+ /* This is either an IDIF object, which identifies objects across
+ * all OSTs, or a regular FID. The IDIF namespace maps legacy
+ * OST objects into the FID namespace. In both cases, we just
+ * pass the FID through, no conversion needed. */
+ if (ostid->oi_fid.f_ver != 0) {
+ CERROR("bad MDT0 id, "DOSTID" ost_idx:%u\n",
+ POSTID(ostid), ost_idx);
+ return -EBADF;
+ }
+ *fid = ostid->oi_fid;
+ }
+
+ return 0;