Fix zvol+btrfs hang

When using a zvol to back a btrfs filesystem the btrfs mount
would hang.  This was due to the bio completion callback used
in btrfs assuming that lower level drivers would never modify
the bio->bi_io_vecs after they were submitted via bio_submit().
If they are modified btrfs will miscalculate which pages need
to be unlocked resulting in a hang.

It's worth mentioning that other file systems such as ext[234]
and xfs work fine because they do not make the same assumption
in the bio completion callback.

The most straight forward way to fix the issue is to present
the semantics expected by btrfs.  This is done by cloning the
bios attached to each request and then using the clones bvecs
to perform the required accounting.  The clones are freed after
each read/write and the original unmodified bios are linked back
in to the request.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #469

module/zfs/dmu.c

@@ -1022,11 +1022,57 @@ dmu_req_copy(void *arg_buf, int size, int *offset, struct request *req)
 	return 0;
 }
 
+static void
+dmu_bio_put(struct bio *bio)
+{
+	struct bio *bio_next;
+
+	while (bio) {
+		bio_next = bio->bi_next;
+		bio_put(bio);
+		bio = bio_next;
+	}
+}
+
+static int
+dmu_bio_clone(struct bio *bio, struct bio **bio_copy)
+{
+	struct bio *bio_root = NULL;
+	struct bio *bio_last = NULL;
+	struct bio *bio_new;
+
+	if (bio == NULL)
+		return EINVAL;
+
+	while (bio) {
+		bio_new = bio_clone(bio, GFP_NOIO);
+		if (bio_new == NULL) {
+			dmu_bio_put(bio_root);
+			return ENOMEM;
+		}
+
+		if (bio_last) {
+			bio_last->bi_next = bio_new;
+			bio_last = bio_new;
+		} else {
+			bio_root = bio_new;
+			bio_last = bio_new;
+		}
+
+		bio = bio->bi_next;
+	}
+
+	*bio_copy = bio_root;
+
+	return 0;
+}
+
 int
 dmu_read_req(objset_t *os, uint64_t object, struct request *req)
 {
 	uint64_t size = blk_rq_bytes(req);
 	uint64_t offset = blk_rq_pos(req) << 9;
+	struct bio *bio_saved = req->bio;
 	dmu_buf_t **dbp;
 	int numbufs, i, err;
 
@@ -1039,6 +1085,10 @@ dmu_read_req(objset_t *os, uint64_t object, struct request *req)
 	if (err)
 		return (err);
 
+	err = dmu_bio_clone(bio_saved, &req->bio);
+	if (err)
+		goto error;
+
 	for (i = 0; i < numbufs; i++) {
 		int tocpy, didcpy, bufoff;
 		dmu_buf_t *db = dbp[i];
@@ -1062,6 +1112,10 @@ dmu_read_req(objset_t *os, uint64_t object, struct request *req)
 		offset += didcpy;
 		err = 0;
 	}
+
+	dmu_bio_put(req->bio);
+	req->bio = bio_saved;
+error:
 	dmu_buf_rele_array(dbp, numbufs, FTAG);
 
 	return (err);
@@ -1072,6 +1126,7 @@ dmu_write_req(objset_t *os, uint64_t object, struct request *req, dmu_tx_t *tx)
 {
 	uint64_t size = blk_rq_bytes(req);
 	uint64_t offset = blk_rq_pos(req) << 9;
+	struct bio *bio_saved = req->bio;
 	dmu_buf_t **dbp;
 	int numbufs;
 	int err = 0;
@@ -1085,6 +1140,10 @@ dmu_write_req(objset_t *os, uint64_t object, struct request *req, dmu_tx_t *tx)
 	if (err)
 		return (err);
 
+	err = dmu_bio_clone(bio_saved, &req->bio);
+	if (err)
+		goto error;
+
 	for (i = 0; i < numbufs; i++) {
 		int tocpy, didcpy, bufoff;
 		dmu_buf_t *db = dbp[i];
@@ -1119,7 +1178,11 @@ dmu_write_req(objset_t *os, uint64_t object, struct request *req, dmu_tx_t *tx)
 		err = 0;
 	}
 
+	dmu_bio_put(req->bio);
+	req->bio = bio_saved;
+error:
 	dmu_buf_rele_array(dbp, numbufs, FTAG);
+
 	return (err);
 }