rbd: new request handling code

The notable changes are:

- instead of explicitly stat'ing the object to see if it exists before
  issuing the write, send the write optimistically along with the stat
  in a single OSD request
- zero copyup optimization
- all object requests are associated with an image request and have
  a valid ->img_request pointer; there are no standalone (!IMG_DATA)
  object requests anymore
- code is structured as a state machine (vs a bunch of callbacks with
  implicit state)

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

drivers/block/rbd.c

@@ -235,11 +235,37 @@ enum obj_req_flags {
 	OBJ_REQ_EXISTS,		/* target exists: no = 0, yes = 1 */
 };
 
+/*
+ * Writes go through the following state machine to deal with
+ * layering:
+ *
+ *                       need copyup
+ * RBD_OBJ_WRITE_GUARD ---------------> RBD_OBJ_WRITE_COPYUP
+ *        |     ^                              |
+ *        v     \------------------------------/
+ *      done
+ *        ^
+ *        |
+ * RBD_OBJ_WRITE_FLAT
+ *
+ * Writes start in RBD_OBJ_WRITE_GUARD or _FLAT, depending on whether
+ * there is a parent or not.
+ */
+enum rbd_obj_write_state {
+	RBD_OBJ_WRITE_FLAT = 1,
+	RBD_OBJ_WRITE_GUARD,
+	RBD_OBJ_WRITE_COPYUP,
+};
+
 struct rbd_obj_request {
 	u64			object_no;
 	u64			offset;		/* object start byte */
 	u64			length;		/* bytes from offset */
 	unsigned long		flags;
+	union {
+		bool			tried_parent;	/* for reads */
+		enum rbd_obj_write_state write_state;	/* for writes */
+	};
 
 	/*
 	 * An object request associated with an image will have its
@@ -1282,6 +1308,27 @@ static void zero_bvecs(struct ceph_bvec_iter *bvec_pos, u32 off, u32 bytes)
 	}));
 }
 
+/*
+ * Zero a range in @obj_req data buffer defined by a bio (list) or
+ * bio_vec array.
+ *
+ * @off is relative to the start of the data buffer.
+ */
+static void rbd_obj_zero_range(struct rbd_obj_request *obj_req, u32 off,
+			       u32 bytes)
+{
+	switch (obj_req->type) {
+	case OBJ_REQUEST_BIO:
+		zero_bios(&obj_req->bio_pos, off, bytes);
+		break;
+	case OBJ_REQUEST_BVECS:
+		zero_bvecs(&obj_req->bvec_pos, off, bytes);
+		break;
+	default:
+		rbd_assert(0);
+	}
+}
+
 /*
  * The default/initial value for all object request flags is 0.  For
  * each flag, once its value is set to 1 it is never reset to 0
@@ -1567,6 +1614,35 @@ rbd_img_request_op_type(struct rbd_img_request *img_request)
 		return OBJ_OP_READ;
 }
 
+static bool rbd_obj_is_entire(struct rbd_obj_request *obj_req)
+{
+	struct rbd_device *rbd_dev = obj_req->img_request->rbd_dev;
+
+	return !obj_req->offset &&
+	       obj_req->length == rbd_dev->layout.object_size;
+}
+
+static bool rbd_obj_is_tail(struct rbd_obj_request *obj_req)
+{
+	struct rbd_device *rbd_dev = obj_req->img_request->rbd_dev;
+
+	return obj_req->offset + obj_req->length ==
+					rbd_dev->layout.object_size;
+}
+
+static bool rbd_img_is_write(struct rbd_img_request *img_req)
+{
+	switch (rbd_img_request_op_type(img_req)) {
+	case OBJ_OP_READ:
+		return false;
+	case OBJ_OP_WRITE:
+	case OBJ_OP_DISCARD:
+		return true;
+	default:
+		rbd_assert(0);
+	}
+}
+
 static void
 rbd_img_obj_request_read_callback(struct rbd_obj_request *obj_request)
 {
@@ -1697,63 +1773,28 @@ static void rbd_osd_call_callback(struct rbd_obj_request *obj_request)
 		obj_request_done_set(obj_request);
 }
 
+static void rbd_obj_handle_request(struct rbd_obj_request *obj_req);
+
 static void rbd_osd_req_callback(struct ceph_osd_request *osd_req)
 {
-	struct rbd_obj_request *obj_request = osd_req->r_priv;
+	struct rbd_obj_request *obj_req = osd_req->r_priv;
-	u16 opcode;
 
-	dout("%s: osd_req %p\n", __func__, osd_req);
+	dout("%s osd_req %p result %d for obj_req %p\n", __func__, osd_req,
-	rbd_assert(osd_req == obj_request->osd_req);
+	     osd_req->r_result, obj_req);
-	if (obj_request_img_data_test(obj_request)) {
+	rbd_assert(osd_req == obj_req->osd_req);
-		rbd_assert(obj_request->img_request);
-		rbd_assert(obj_request->which != BAD_WHICH);
-	} else {
-		rbd_assert(obj_request->which == BAD_WHICH);
-	}
-
-	if (osd_req->r_result < 0)
-		obj_request->result = osd_req->r_result;
 
+	obj_req->result = osd_req->r_result < 0 ? osd_req->r_result : 0;
+	if (!obj_req->result && !rbd_img_is_write(obj_req->img_request))
+		obj_req->xferred = osd_req->r_result;
+	else
 		/*
-	 * We support a 64-bit length, but ultimately it has to be
+		 * Writes aren't allowed to return a data payload.  In some
-	 * passed to the block layer, which just supports a 32-bit
+		 * guarded write cases (e.g. stat + zero on an empty object)
-	 * length field.
+		 * a stat response makes it through, but we don't care.
 		 */
-	obj_request->xferred = osd_req->r_ops[0].outdata_len;
+		obj_req->xferred = 0;
-	rbd_assert(obj_request->xferred < (u64)UINT_MAX);
 
-	opcode = osd_req->r_ops[0].op;
+	rbd_obj_handle_request(obj_req);
-	switch (opcode) {
-	case CEPH_OSD_OP_READ:
-		rbd_osd_read_callback(obj_request);
-		break;
-	case CEPH_OSD_OP_SETALLOCHINT:
-		rbd_assert(osd_req->r_ops[1].op == CEPH_OSD_OP_WRITE ||
-			   osd_req->r_ops[1].op == CEPH_OSD_OP_WRITEFULL);
-		/* fall through */
-	case CEPH_OSD_OP_WRITE:
-	case CEPH_OSD_OP_WRITEFULL:
-		rbd_osd_write_callback(obj_request);
-		break;
-	case CEPH_OSD_OP_STAT:
-		rbd_osd_stat_callback(obj_request);
-		break;
-	case CEPH_OSD_OP_DELETE:
-	case CEPH_OSD_OP_TRUNCATE:
-	case CEPH_OSD_OP_ZERO:
-		rbd_osd_discard_callback(obj_request);
-		break;
-	case CEPH_OSD_OP_CALL:
-		rbd_osd_call_callback(obj_request);
-		break;
-	default:
-		rbd_warn(NULL, "unexpected OSD op: object_no %016llx opcode %d",
-			 obj_request->object_no, opcode);
-		break;
-	}
-
-	if (obj_request_done_test(obj_request))
-		rbd_obj_request_complete(obj_request);
 }
 
 static void rbd_osd_req_format_read(struct rbd_obj_request *obj_request)
@@ -1806,12 +1847,6 @@ __rbd_osd_req_create(struct rbd_device *rbd_dev,
 	return NULL;
 }
 
-/*
- * Create an osd request.  A read request has one osd op (read).
- * A write request has either one (watch) or two (hint+write) osd ops.
- * (All rbd data writes are prefixed with an allocation hint op, but
- * technically osd watch is a write request, hence this distinction.)
- */
 static struct ceph_osd_request *rbd_osd_req_create(
 					struct rbd_device *rbd_dev,
 					enum obj_operation_type op_type,
@@ -1831,8 +1866,6 @@ static struct ceph_osd_request *rbd_osd_req_create(
 		snapc = img_request->snapc;
 	}
 
-	rbd_assert(num_ops == 1 || ((op_type == OBJ_OP_WRITE) && num_ops == 2));
-
 	return __rbd_osd_req_create(rbd_dev, snapc, num_ops,
 	    (op_type == OBJ_OP_WRITE || op_type == OBJ_OP_DISCARD) ?
 	    CEPH_OSD_FLAG_WRITE : CEPH_OSD_FLAG_READ, obj_request);
@@ -2251,6 +2284,211 @@ static void rbd_img_obj_request_fill(struct rbd_obj_request *obj_request,
 		rbd_osd_req_format_read(obj_request);
 }
 
+static void rbd_osd_req_setup_data(struct rbd_obj_request *obj_req, u32 which)
+{
+	switch (obj_req->type) {
+	case OBJ_REQUEST_BIO:
+		osd_req_op_extent_osd_data_bio(obj_req->osd_req, which,
+					       &obj_req->bio_pos,
+					       obj_req->length);
+		break;
+	case OBJ_REQUEST_BVECS:
+		rbd_assert(obj_req->bvec_pos.iter.bi_size ==
+							obj_req->length);
+		osd_req_op_extent_osd_data_bvec_pos(obj_req->osd_req, which,
+						    &obj_req->bvec_pos);
+		break;
+	default:
+		rbd_assert(0);
+	}
+}
+
+static int rbd_obj_setup_read(struct rbd_obj_request *obj_req)
+{
+	struct rbd_device *rbd_dev = obj_req->img_request->rbd_dev;
+
+	obj_req->osd_req = rbd_osd_req_create(rbd_dev, OBJ_OP_READ, 1, obj_req);
+	if (!obj_req->osd_req)
+		return -ENOMEM;
+
+	osd_req_op_extent_init(obj_req->osd_req, 0, CEPH_OSD_OP_READ,
+			       obj_req->offset, obj_req->length, 0, 0);
+	rbd_osd_req_setup_data(obj_req, 0);
+
+	rbd_osd_req_format_read(obj_req);
+	return 0;
+}
+
+static int __rbd_obj_setup_stat(struct rbd_obj_request *obj_req,
+				unsigned int which)
+{
+	struct page **pages;
+
+	/*
+	 * The response data for a STAT call consists of:
+	 *     le64 length;
+	 *     struct {
+	 *         le32 tv_sec;
+	 *         le32 tv_nsec;
+	 *     } mtime;
+	 */
+	pages = ceph_alloc_page_vector(1, GFP_NOIO);
+	if (IS_ERR(pages))
+		return PTR_ERR(pages);
+
+	osd_req_op_init(obj_req->osd_req, which, CEPH_OSD_OP_STAT, 0);
+	osd_req_op_raw_data_in_pages(obj_req->osd_req, which, pages,
+				     8 + sizeof(struct ceph_timespec),
+				     0, false, true);
+	return 0;
+}
+
+static void __rbd_obj_setup_write(struct rbd_obj_request *obj_req,
+				  unsigned int which)
+{
+	struct rbd_device *rbd_dev = obj_req->img_request->rbd_dev;
+	u16 opcode;
+
+	osd_req_op_alloc_hint_init(obj_req->osd_req, which++,
+				   rbd_dev->layout.object_size,
+				   rbd_dev->layout.object_size);
+
+	if (rbd_obj_is_entire(obj_req))
+		opcode = CEPH_OSD_OP_WRITEFULL;
+	else
+		opcode = CEPH_OSD_OP_WRITE;
+
+	osd_req_op_extent_init(obj_req->osd_req, which, opcode,
+			       obj_req->offset, obj_req->length, 0, 0);
+	rbd_osd_req_setup_data(obj_req, which++);
+
+	rbd_assert(which == obj_req->osd_req->r_num_ops);
+	rbd_osd_req_format_write(obj_req);
+}
+
+static int rbd_obj_setup_write(struct rbd_obj_request *obj_req)
+{
+	struct rbd_device *rbd_dev = obj_req->img_request->rbd_dev;
+	unsigned int num_osd_ops, which = 0;
+	int ret;
+
+	if (obj_request_overlaps_parent(obj_req)) {
+		obj_req->write_state = RBD_OBJ_WRITE_GUARD;
+		num_osd_ops = 3; /* stat + setallochint + write/writefull */
+	} else {
+		obj_req->write_state = RBD_OBJ_WRITE_FLAT;
+		num_osd_ops = 2; /* setallochint + write/writefull */
+	}
+
+	obj_req->osd_req = rbd_osd_req_create(rbd_dev, OBJ_OP_WRITE,
+					      num_osd_ops, obj_req);
+	if (!obj_req->osd_req)
+		return -ENOMEM;
+
+	if (obj_request_overlaps_parent(obj_req)) {
+		ret = __rbd_obj_setup_stat(obj_req, which++);
+		if (ret)
+			return ret;
+	}
+
+	__rbd_obj_setup_write(obj_req, which);
+	return 0;
+}
+
+static void __rbd_obj_setup_discard(struct rbd_obj_request *obj_req,
+				    unsigned int which)
+{
+	u16 opcode;
+
+	if (rbd_obj_is_entire(obj_req)) {
+		if (obj_request_overlaps_parent(obj_req)) {
+			opcode = CEPH_OSD_OP_TRUNCATE;
+		} else {
+			osd_req_op_init(obj_req->osd_req, which++,
+					CEPH_OSD_OP_DELETE, 0);
+			opcode = 0;
+		}
+	} else if (rbd_obj_is_tail(obj_req)) {
+		opcode = CEPH_OSD_OP_TRUNCATE;
+	} else {
+		opcode = CEPH_OSD_OP_ZERO;
+	}
+
+	if (opcode)
+		osd_req_op_extent_init(obj_req->osd_req, which++, opcode,
+				       obj_req->offset, obj_req->length,
+				       0, 0);
+
+	rbd_assert(which == obj_req->osd_req->r_num_ops);
+	rbd_osd_req_format_write(obj_req);
+}
+
+static int rbd_obj_setup_discard(struct rbd_obj_request *obj_req)
+{
+	struct rbd_device *rbd_dev = obj_req->img_request->rbd_dev;
+	unsigned int num_osd_ops, which = 0;
+	int ret;
+
+	if (rbd_obj_is_entire(obj_req)) {
+		obj_req->write_state = RBD_OBJ_WRITE_FLAT;
+		num_osd_ops = 1; /* truncate/delete */
+	} else {
+		if (obj_request_overlaps_parent(obj_req)) {
+			obj_req->write_state = RBD_OBJ_WRITE_GUARD;
+			num_osd_ops = 2; /* stat + truncate/zero */
+		} else {
+			obj_req->write_state = RBD_OBJ_WRITE_FLAT;
+			num_osd_ops = 1; /* truncate/zero */
+		}
+	}
+
+	obj_req->osd_req = rbd_osd_req_create(rbd_dev, OBJ_OP_DISCARD,
+					      num_osd_ops, obj_req);
+	if (!obj_req->osd_req)
+		return -ENOMEM;
+
+	if (!rbd_obj_is_entire(obj_req) &&
+	    obj_request_overlaps_parent(obj_req)) {
+		ret = __rbd_obj_setup_stat(obj_req, which++);
+		if (ret)
+			return ret;
+	}
+
+	__rbd_obj_setup_discard(obj_req, which);
+	return 0;
+}
+
+/*
+ * For each object request in @img_req, allocate an OSD request, add
+ * individual OSD ops and prepare them for submission.  The number of
+ * OSD ops depends on op_type and the overlap point (if any).
+ */
+static int __rbd_img_fill_request(struct rbd_img_request *img_req)
+{
+	struct rbd_obj_request *obj_req;
+	int ret;
+
+	for_each_obj_request(img_req, obj_req) {
+		switch (rbd_img_request_op_type(img_req)) {
+		case OBJ_OP_READ:
+			ret = rbd_obj_setup_read(obj_req);
+			break;
+		case OBJ_OP_WRITE:
+			ret = rbd_obj_setup_write(obj_req);
+			break;
+		case OBJ_OP_DISCARD:
+			ret = rbd_obj_setup_discard(obj_req);
+			break;
+		default:
+			rbd_assert(0);
+		}
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
 /*
  * Split up an image request into one or more object requests, each
  * to a different object.  The "type" parameter indicates whether
@@ -2268,7 +2506,6 @@ static int rbd_img_request_fill(struct rbd_img_request *img_request,
 	struct rbd_obj_request *next_obj_request;
 	struct ceph_bio_iter bio_it;
 	struct ceph_bvec_iter bvec_it;
-	enum obj_operation_type op_type;
 	u64 img_offset;
 	u64 resid;
 
@@ -2278,7 +2515,6 @@ static int rbd_img_request_fill(struct rbd_img_request *img_request,
 	img_offset = img_request->offset;
 	resid = img_request->length;
 	rbd_assert(resid > 0);
-	op_type = rbd_img_request_op_type(img_request);
 
 	if (type == OBJ_REQUEST_BIO) {
 		bio_it = *(struct ceph_bio_iter *)data_desc;
@@ -2289,7 +2525,6 @@ static int rbd_img_request_fill(struct rbd_img_request *img_request,
 	}
 
 	while (resid) {
-		struct ceph_osd_request *osd_req;
 		u64 object_no = img_offset >> rbd_dev->header.obj_order;
 		u64 offset = rbd_segment_offset(rbd_dev, img_offset);
 		u64 length = rbd_segment_length(rbd_dev, img_offset, resid);
@@ -2317,23 +2552,14 @@ static int rbd_img_request_fill(struct rbd_img_request *img_request,
 			ceph_bvec_iter_advance(&bvec_it, length);
 		}
 
-		osd_req = rbd_osd_req_create(rbd_dev, op_type,
-					(op_type == OBJ_OP_WRITE) ? 2 : 1,
-					obj_request);
-		if (!osd_req)
-			goto out_unwind;
-
-		obj_request->osd_req = osd_req;
 		obj_request->callback = rbd_img_obj_callback;
 		obj_request->img_offset = img_offset;
 
-		rbd_img_obj_request_fill(obj_request, osd_req, op_type, 0);
-
 		img_offset += length;
 		resid -= length;
 	}
 
-	return 0;
+	return __rbd_img_fill_request(img_request);
 
 out_unwind:
 	for_each_obj_request_safe(img_request, obj_request, next_obj_request)
@@ -2712,16 +2938,171 @@ static int rbd_img_request_submit(struct rbd_img_request *img_request)
 
 	rbd_img_request_get(img_request);
 	for_each_obj_request_safe(img_request, obj_request, next_obj_request) {
-		ret = rbd_img_obj_request_submit(obj_request);
+		rbd_obj_request_submit(obj_request);
-		if (ret)
-			goto out_put_ireq;
 	}
 
-out_put_ireq:
 	rbd_img_request_put(img_request);
 	return ret;
 }
 
+static void rbd_img_end_child_request(struct rbd_img_request *img_req);
+
+static int rbd_obj_read_from_parent(struct rbd_obj_request *obj_req,
+				    u64 img_offset, u32 bytes)
+{
+	struct rbd_img_request *img_req = obj_req->img_request;
+	struct rbd_img_request *child_img_req;
+	int ret;
+
+	child_img_req = rbd_parent_request_create(obj_req, img_offset, bytes);
+	if (!child_img_req)
+		return -ENOMEM;
+
+	child_img_req->callback = rbd_img_end_child_request;
+
+	if (!rbd_img_is_write(img_req)) {
+		switch (obj_req->type) {
+		case OBJ_REQUEST_BIO:
+			ret = rbd_img_request_fill(child_img_req,
+						   OBJ_REQUEST_BIO,
+						   &obj_req->bio_pos);
+			break;
+		case OBJ_REQUEST_BVECS:
+			ret = rbd_img_request_fill(child_img_req,
+						   OBJ_REQUEST_BVECS,
+						   &obj_req->bvec_pos);
+			break;
+		default:
+			rbd_assert(0);
+		}
+	} else {
+		struct ceph_bvec_iter it = {
+			.bvecs = obj_req->copyup_bvecs,
+			.iter = { .bi_size = bytes },
+		};
+
+		ret = rbd_img_request_fill(child_img_req, OBJ_REQUEST_BVECS,
+					   &it);
+	}
+	if (ret) {
+		rbd_img_request_put(child_img_req);
+		return ret;
+	}
+
+	rbd_img_request_submit(child_img_req);
+	return 0;
+}
+
+static bool rbd_obj_handle_read(struct rbd_obj_request *obj_req)
+{
+	struct rbd_device *rbd_dev = obj_req->img_request->rbd_dev;
+	int ret;
+
+	if (obj_req->result == -ENOENT &&
+	    obj_req->img_offset < rbd_dev->parent_overlap &&
+	    !obj_req->tried_parent) {
+		u64 obj_overlap = min(obj_req->length,
+			      rbd_dev->parent_overlap - obj_req->img_offset);
+
+		obj_req->tried_parent = true;
+		ret = rbd_obj_read_from_parent(obj_req, obj_req->img_offset,
+					       obj_overlap);
+		if (ret) {
+			obj_req->result = ret;
+			return true;
+		}
+		return false;
+	}
+
+	/*
+	 * -ENOENT means a hole in the image -- zero-fill the entire
+	 * length of the request.  A short read also implies zero-fill
+	 * to the end of the request.  In both cases we update xferred
+	 * count to indicate the whole request was satisfied.
+	 */
+	if (obj_req->result == -ENOENT ||
+	    (!obj_req->result && obj_req->xferred < obj_req->length)) {
+		rbd_assert(!obj_req->xferred || !obj_req->result);
+		rbd_obj_zero_range(obj_req, obj_req->xferred,
+				   obj_req->length - obj_req->xferred);
+		obj_req->result = 0;
+		obj_req->xferred = obj_req->length;
+	}
+
+	return true;
+}
+
+/*
+ * copyup_bvecs pages are never highmem pages
+ */
+static bool is_zero_bvecs(struct bio_vec *bvecs, u32 bytes)
+{
+	struct ceph_bvec_iter it = {
+		.bvecs = bvecs,
+		.iter = { .bi_size = bytes },
+	};
+
+	ceph_bvec_iter_advance_step(&it, bytes, ({
+		if (memchr_inv(page_address(bv.bv_page) + bv.bv_offset, 0,
+			       bv.bv_len))
+			return false;
+	}));
+	return true;
+}
+
+static int rbd_obj_issue_copyup(struct rbd_obj_request *obj_req, u32 bytes)
+{
+	struct rbd_device *rbd_dev = obj_req->img_request->rbd_dev;
+	unsigned int num_osd_ops = obj_req->osd_req->r_num_ops;
+
+	dout("%s obj_req %p bytes %u\n", __func__, obj_req, bytes);
+	rbd_assert(obj_req->osd_req->r_ops[0].op == CEPH_OSD_OP_STAT);
+	rbd_osd_req_destroy(obj_req->osd_req);
+
+	/*
+	 * Create a copyup request with the same number of OSD ops as
+	 * the original request.  The original request was stat + op(s),
+	 * the new copyup request will be copyup + the same op(s).
+	 */
+	obj_req->osd_req = rbd_osd_req_create(rbd_dev,
+				rbd_img_request_op_type(obj_req->img_request),
+				num_osd_ops, obj_req);
+	if (!obj_req->osd_req)
+		return -ENOMEM;
+
+	/*
+	 * Only send non-zero copyup data to save some I/O and network
+	 * bandwidth -- zero copyup data is equivalent to the object not
+	 * existing.
+	 */
+	if (is_zero_bvecs(obj_req->copyup_bvecs, bytes)) {
+		dout("%s obj_req %p detected zeroes\n", __func__, obj_req);
+		bytes = 0;
+	}
+
+	osd_req_op_cls_init(obj_req->osd_req, 0, CEPH_OSD_OP_CALL, "rbd",
+			    "copyup");
+	osd_req_op_cls_request_data_bvecs(obj_req->osd_req, 0,
+					  obj_req->copyup_bvecs, bytes);
+
+	switch (rbd_img_request_op_type(obj_req->img_request)) {
+	case OBJ_OP_WRITE:
+		__rbd_obj_setup_write(obj_req, 1);
+		break;
+	case OBJ_OP_DISCARD:
+		rbd_assert(!rbd_obj_is_entire(obj_req));
+		__rbd_obj_setup_discard(obj_req, 1);
+		break;
+	default:
+		rbd_assert(0);
+	}
+
+	rbd_obj_request_submit(obj_req);
+	/* FIXME: in lieu of rbd_img_obj_callback() */
+	rbd_img_request_put(obj_req->img_request);
+	return 0;
+}
+
 static int setup_copyup_bvecs(struct rbd_obj_request *obj_req, u64 obj_overlap)
 {
 	u32 i;
@@ -2850,6 +3231,149 @@ static void rbd_img_parent_read(struct rbd_obj_request *obj_request)
 	obj_request_done_set(obj_request);
 }
 
+static int rbd_obj_handle_write_guard(struct rbd_obj_request *obj_req)
+{
+	struct rbd_device *rbd_dev = obj_req->img_request->rbd_dev;
+	u64 img_offset;
+	u64 obj_overlap;
+	int ret;
+
+	if (!obj_request_overlaps_parent(obj_req)) {
+		/*
+		 * The overlap has become 0 (most likely because the
+		 * image has been flattened).  Use rbd_obj_issue_copyup()
+		 * to re-submit the original write request -- the copyup
+		 * operation itself will be a no-op, since someone must
+		 * have populated the child object while we weren't
+		 * looking.  Move to WRITE_FLAT state as we'll be done
+		 * with the operation once the null copyup completes.
+		 */
+		obj_req->write_state = RBD_OBJ_WRITE_FLAT;
+		return rbd_obj_issue_copyup(obj_req, 0);
+	}
+
+	/*
+	 * Determine the byte range covered by the object in the
+	 * child image to which the original request was to be sent.
+	 */
+	img_offset = obj_req->img_offset - obj_req->offset;
+	obj_overlap = rbd_dev->layout.object_size;
+
+	/*
+	 * There is no defined parent data beyond the parent
+	 * overlap, so limit what we read at that boundary if
+	 * necessary.
+	 */
+	if (img_offset + obj_overlap > rbd_dev->parent_overlap) {
+		rbd_assert(img_offset < rbd_dev->parent_overlap);
+		obj_overlap = rbd_dev->parent_overlap - img_offset;
+	}
+
+	ret = setup_copyup_bvecs(obj_req, obj_overlap);
+	if (ret)
+		return ret;
+
+	obj_req->write_state = RBD_OBJ_WRITE_COPYUP;
+	return rbd_obj_read_from_parent(obj_req, img_offset, obj_overlap);
+}
+
+static bool rbd_obj_handle_write(struct rbd_obj_request *obj_req)
+{
+	int ret;
+
+again:
+	switch (obj_req->write_state) {
+	case RBD_OBJ_WRITE_GUARD:
+		rbd_assert(!obj_req->xferred);
+		if (obj_req->result == -ENOENT) {
+			/*
+			 * The target object doesn't exist.  Read the data for
+			 * the entire target object up to the overlap point (if
+			 * any) from the parent, so we can use it for a copyup.
+			 */
+			ret = rbd_obj_handle_write_guard(obj_req);
+			if (ret) {
+				obj_req->result = ret;
+				return true;
+			}
+			return false;
+		}
+		/* fall through */
+	case RBD_OBJ_WRITE_FLAT:
+		if (!obj_req->result)
+			/*
+			 * There is no such thing as a successful short
+			 * write -- indicate the whole request was satisfied.
+			 */
+			obj_req->xferred = obj_req->length;
+		return true;
+	case RBD_OBJ_WRITE_COPYUP:
+		obj_req->write_state = RBD_OBJ_WRITE_GUARD;
+		if (obj_req->result)
+			goto again;
+
+		rbd_assert(obj_req->xferred);
+		ret = rbd_obj_issue_copyup(obj_req, obj_req->xferred);
+		if (ret) {
+			obj_req->result = ret;
+			return true;
+		}
+		return false;
+	default:
+		rbd_assert(0);
+	}
+}
+
+/*
+ * Returns true if @obj_req is completed, or false otherwise.
+ */
+static bool __rbd_obj_handle_request(struct rbd_obj_request *obj_req)
+{
+	switch (rbd_img_request_op_type(obj_req->img_request)) {
+	case OBJ_OP_READ:
+		return rbd_obj_handle_read(obj_req);
+	case OBJ_OP_WRITE:
+		return rbd_obj_handle_write(obj_req);
+	case OBJ_OP_DISCARD:
+		if (rbd_obj_handle_write(obj_req)) {
+			/*
+			 * Hide -ENOENT from delete/truncate/zero -- discarding
+			 * a non-existent object is not a problem.
+			 */
+			if (obj_req->result == -ENOENT) {
+				obj_req->result = 0;
+				obj_req->xferred = obj_req->length;
+			}
+			return true;
+		}
+		return false;
+	default:
+		rbd_assert(0);
+	}
+}
+
+static void rbd_img_end_child_request(struct rbd_img_request *img_req)
+{
+	struct rbd_obj_request *obj_req = img_req->obj_request;
+
+	rbd_assert(test_bit(IMG_REQ_CHILD, &img_req->flags));
+
+	obj_req->result = img_req->result;
+	obj_req->xferred = img_req->xferred;
+	rbd_img_request_put(img_req);
+
+	rbd_obj_handle_request(obj_req);
+}
+
+static void rbd_obj_handle_request(struct rbd_obj_request *obj_req)
+{
+	if (!__rbd_obj_handle_request(obj_req))
+		return;
+
+	obj_request_done_set(obj_req);
+	rbd_obj_request_complete(obj_req);
+}
+
 static const struct rbd_client_id rbd_empty_cid;
 
 static bool rbd_cid_equal(const struct rbd_client_id *lhs,