libceph: new sparse_read op, support sparse reads on msgr2 crc codepath

Add support for a new sparse_read ceph_connection operation. The idea is
that the client driver can define this operation use it to do special
handling for incoming reads.

The alloc_msg routine will look at the request and determine whether the
reply is expected to be sparse. If it is, then we'll dispatch to a
different set of state machine states that will repeatedly call the
driver's sparse_read op to get length and placement info for reading the
extent map, and the extents themselves.

This necessitates adding some new field to some other structs:

- The msg gets a new bool to track whether it's a sparse_read request.

- A new field is added to the cursor to track the amount remaining in the
current extent. This is used to cap the read from the socket into the
msg_data

- Handing a revoke with all of this is particularly difficult, so I've
added a new data_len_remain field to the v2 connection info, and then
use that to skip that much on a revoke. We may want to expand the use of
that to the normal read path as well, just for consistency's sake.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-and-tested-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

include/linux/ceph/messenger.h

@@ -17,6 +17,7 @@
 
 struct ceph_msg;
 struct ceph_connection;
+struct ceph_msg_data_cursor;
 
 /*
  * Ceph defines these callbacks for handling connection events.
@@ -70,6 +71,30 @@ struct ceph_connection_operations {
 				      int used_proto, int result,
 				      const int *allowed_protos, int proto_cnt,
 				      const int *allowed_modes, int mode_cnt);
+
+	/**
+	 * sparse_read: read sparse data
+	 * @con: connection we're reading from
+	 * @cursor: data cursor for reading extents
+	 * @buf: optional buffer to read into
+	 *
+	 * This should be called more than once, each time setting up to
+	 * receive an extent into the current cursor position, and zeroing
+	 * the holes between them.
+	 *
+	 * Returns amount of data to be read (in bytes), 0 if reading is
+	 * complete, or -errno if there was an error.
+	 *
+	 * If @buf is set on a >0 return, then the data should be read into
+	 * the provided buffer. Otherwise, it should be read into the cursor.
+	 *
+	 * The sparse read operation is expected to initialize the cursor
+	 * with a length covering up to the end of the last extent.
+	 */
+	int (*sparse_read)(struct ceph_connection *con,
+			   struct ceph_msg_data_cursor *cursor,
+			   char **buf);
+
 };
 
 /* use format string %s%lld */
@@ -207,6 +232,7 @@ struct ceph_msg_data_cursor {
 
 	struct ceph_msg_data	*data;		/* current data item */
 	size_t			resid;		/* bytes not yet consumed */
+	int			sr_resid;	/* residual sparse_read len */
 	bool			need_crc;	/* crc update needed */
 	union {
 #ifdef CONFIG_BLOCK
@@ -251,6 +277,7 @@ struct ceph_msg {
 	struct kref kref;
 	bool more_to_follow;
 	bool needs_out_seq;
+	bool sparse_read;
 	int front_alloc_len;
 
 	struct ceph_msgpool *pool;
@@ -395,6 +422,7 @@ struct ceph_connection_v2_info {
 
 	void *conn_bufs[16];
 	int conn_buf_cnt;
+	int data_len_remain;
 
 	struct kvec in_sign_kvecs[8];
 	struct kvec out_sign_kvecs[8];