libceph: add sparse read support to OSD client

Have get_reply check for the presence of sparse read ops in the
request and set the sparse_read boolean in the msg. That will queue the
messenger layer to use the sparse read codepath instead of the normal
data receive.

Add a new sparse_read operation for the OSD client, driven by its own
state machine. The messenger will repeatedly call the sparse_read
operation, and it will pass back the necessary info to set up to read
the next extent of data, while zero-filling the sparse regions.

The state machine will stop at the end of the last extent, and will
attach the extent map buffer to the ceph_osd_req_op so that the caller
can use it.

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/osd_client.h

@@ -40,6 +40,36 @@ struct ceph_sparse_extent {
 	u64	len;
 } __packed;
 
+/* Sparse read state machine state values */
+enum ceph_sparse_read_state {
+	CEPH_SPARSE_READ_HDR	= 0,
+	CEPH_SPARSE_READ_EXTENTS,
+	CEPH_SPARSE_READ_DATA_LEN,
+	CEPH_SPARSE_READ_DATA,
+};
+
+/*
+ * A SPARSE_READ reply is a 32-bit count of extents, followed by an array of
+ * 64-bit offset/length pairs, and then all of the actual file data
+ * concatenated after it (sans holes).
+ *
+ * Unfortunately, we don't know how long the extent array is until we've
+ * started reading the data section of the reply. The caller should send down
+ * a destination buffer for the array, but we'll alloc one if it's too small
+ * or if the caller doesn't.
+ */
+struct ceph_sparse_read {
+	enum ceph_sparse_read_state	sr_state;    /* state machine state */
+	u64				sr_req_off;  /* orig request offset */
+	u64				sr_req_len;  /* orig request length */
+	u64				sr_pos;      /* current pos in buffer */
+	int				sr_index;    /* current extent index */
+	__le32				sr_datalen;  /* length of actual data */
+	u32				sr_count;    /* extent count in reply */
+	int				sr_ext_len;  /* length of extent array */
+	struct ceph_sparse_extent	*sr_extent;  /* extent array */
+};
+
 /*
  * A given osd we're communicating with.
  *
@@ -48,6 +78,7 @@ struct ceph_sparse_extent {
  */
 struct ceph_osd {
 	refcount_t o_ref;
+	int o_sparse_op_idx;
 	struct ceph_osd_client *o_osdc;
 	int o_osd;
 	int o_incarnation;
@@ -63,6 +94,7 @@ struct ceph_osd {
 	unsigned long lru_ttl;
 	struct list_head o_keepalive_item;
 	struct mutex lock;
+	struct ceph_sparse_read	o_sparse_read;
 };
 
 #define CEPH_OSD_SLAB_OPS	2