libceph: add new iov_iter-based ceph_msg_data_type and ceph_osd_data_type

Add an iov_iter to the unions in ceph_msg_data and ceph_msg_data_cursor.
Instead of requiring a list of pages or bvecs, we can just use an
iov_iter directly, and avoid extra allocations.

We assume that the pages represented by the iter are pinned such that
they shouldn't incur page faults, which is the case for the iov_iters
created by netfs.

While working on this, Al Viro informed me that he was going to change
iov_iter_get_pages to auto-advance the iterator as that pattern is more
or less required for ITER_PIPE anyway. We emulate that here for now by
advancing in the _next op and tracking that amount in the "lastlen"
field.

In the event that _next is called twice without an intervening
_advance, we revert the iov_iter by the remaining lastlen before
calling iov_iter_get_pages.

Cc: Al Viro <[e-mail]>
Cc: David Howells <[e-mail]>
Signed-off-by: Jeff Layton <[e-mail]>
Reviewed-by: Xiubo Li <[e-mail]>
Reviewed-and-tested-by: Luís Henriques <[e-mail]>
Reviewed-by: Milind Changire <[e-mail]>
Signed-off-by: Ilya Dryomov <[e-mail]>

include/linux/ceph/messenger.h

···
 	CEPH_MSG_DATA_BIO,	/* data source/destination is a bio list */
 #endif /* CONFIG_BLOCK */
 	CEPH_MSG_DATA_BVECS,	/* data source/destination is a bio_vec array */
+	CEPH_MSG_DATA_ITER,	/* data source/destination is an iov_iter */
 };
 
 #ifdef CONFIG_BLOCK
···
 			bool		own_pages;
 		};
 		struct ceph_pagelist	*pagelist;
+		struct iov_iter		iter;
 	};
 };
 
···
 		struct {				/* pagelist */
 			struct page	*page;		/* page from list */
 			size_t		offset;		/* bytes from list */
+		};
+		struct {
+			struct iov_iter		iov_iter;
+			unsigned int		lastlen;
 		};
 	};
 };
···
 #endif /* CONFIG_BLOCK */
 void ceph_msg_data_add_bvecs(struct ceph_msg *msg,
 			     struct ceph_bvec_iter *bvec_pos);
+void ceph_msg_data_add_iter(struct ceph_msg *msg,
+			    struct iov_iter *iter);
 
 struct ceph_msg *ceph_msg_new2(int type, int front_len, int max_data_items,
 			       gfp_t flags, bool can_fail);

include/linux/ceph/osd_client.h

···
 	CEPH_OSD_DATA_TYPE_BIO,
 #endif /* CONFIG_BLOCK */
 	CEPH_OSD_DATA_TYPE_BVECS,
+	CEPH_OSD_DATA_TYPE_ITER,
 };
 
 struct ceph_osd_data {
···
 			struct ceph_bvec_iter	bvec_pos;
 			u32			num_bvecs;
 		};
+		struct iov_iter		iter;
 	};
 };
 
···
 void osd_req_op_extent_osd_data_bvec_pos(struct ceph_osd_request *osd_req,
 					 unsigned int which,
 					 struct ceph_bvec_iter *bvec_pos);
+void osd_req_op_extent_osd_iter(struct ceph_osd_request *osd_req,
+				unsigned int which, struct iov_iter *iter);
 
 extern void osd_req_op_cls_request_data_pagelist(struct ceph_osd_request *,
 					unsigned int which,

net/ceph/messenger.c

···
 	return true;
 }
 
+static void ceph_msg_data_iter_cursor_init(struct ceph_msg_data_cursor *cursor,
+					   size_t length)
+{
+	struct ceph_msg_data *data = cursor->data;
+
+	cursor->iov_iter = data->iter;
+	cursor->lastlen = 0;
+	iov_iter_truncate(&cursor->iov_iter, length);
+	cursor->resid = iov_iter_count(&cursor->iov_iter);
+}
+
+static struct page *ceph_msg_data_iter_next(struct ceph_msg_data_cursor *cursor,
+					    size_t *page_offset, size_t *length)
+{
+	struct page *page;
+	ssize_t len;
+
+	if (cursor->lastlen)
+		iov_iter_revert(&cursor->iov_iter, cursor->lastlen);
+
+	len = iov_iter_get_pages2(&cursor->iov_iter, &page, PAGE_SIZE,
+				  1, page_offset);
+	BUG_ON(len < 0);
+
+	cursor->lastlen = len;
+
+	/*
+	 * FIXME: The assumption is that the pages represented by the iov_iter
+	 *	  are pinned, with the references held by the upper-level
+	 *	  callers, or by virtue of being under writeback. Eventually,
+	 *	  we'll get an iov_iter_get_pages2 variant that doesn't take
+	 *	  page refs. Until then, just put the page ref.
+	 */
+	VM_BUG_ON_PAGE(!PageWriteback(page) && page_count(page) < 2, page);
+	put_page(page);
+
+	*length = min_t(size_t, len, cursor->resid);
+	return page;
+}
+
+static bool ceph_msg_data_iter_advance(struct ceph_msg_data_cursor *cursor,
+				       size_t bytes)
+{
+	BUG_ON(bytes > cursor->resid);
+	cursor->resid -= bytes;
+
+	if (bytes < cursor->lastlen) {
+		cursor->lastlen -= bytes;
+	} else {
+		iov_iter_advance(&cursor->iov_iter, bytes - cursor->lastlen);
+		cursor->lastlen = 0;
+	}
+
+	return cursor->resid;
+}
+
 /*
  * Message data is handled (sent or received) in pieces, where each
  * piece resides on a single page.  The network layer might not
···
 #endif /* CONFIG_BLOCK */
 	case CEPH_MSG_DATA_BVECS:
 		ceph_msg_data_bvecs_cursor_init(cursor, length);
+		break;
+	case CEPH_MSG_DATA_ITER:
+		ceph_msg_data_iter_cursor_init(cursor, length);
 		break;
 	case CEPH_MSG_DATA_NONE:
 	default:
···
 	case CEPH_MSG_DATA_BVECS:
 		page = ceph_msg_data_bvecs_next(cursor, page_offset, length);
 		break;
+	case CEPH_MSG_DATA_ITER:
+		page = ceph_msg_data_iter_next(cursor, page_offset, length);
+		break;
 	case CEPH_MSG_DATA_NONE:
 	default:
 		page = NULL;
···
 #endif /* CONFIG_BLOCK */
 	case CEPH_MSG_DATA_BVECS:
 		new_piece = ceph_msg_data_bvecs_advance(cursor, bytes);
+		break;
+	case CEPH_MSG_DATA_ITER:
+		new_piece = ceph_msg_data_iter_advance(cursor, bytes);
 		break;
 	case CEPH_MSG_DATA_NONE:
 	default:
···
 	msg->data_length += bvec_pos->iter.bi_size;
 }
 EXPORT_SYMBOL(ceph_msg_data_add_bvecs);
+
+void ceph_msg_data_add_iter(struct ceph_msg *msg,
+			    struct iov_iter *iter)
+{
+	struct ceph_msg_data *data;
+
+	data = ceph_msg_data_add(msg);
+	data->type = CEPH_MSG_DATA_ITER;
+	data->iter = *iter;
+
+	msg->data_length += iov_iter_count(&data->iter);
+}
 
 /*
  * construct a new message with given type, size

net/ceph/osd_client.c

···
 	osd_data->num_bvecs = num_bvecs;
 }
 
+static void ceph_osd_iter_init(struct ceph_osd_data *osd_data,
+			       struct iov_iter *iter)
+{
+	osd_data->type = CEPH_OSD_DATA_TYPE_ITER;
+	osd_data->iter = *iter;
+}
+
 static struct ceph_osd_data *
 osd_req_op_raw_data_in(struct ceph_osd_request *osd_req, unsigned int which)
 {
···
 }
 EXPORT_SYMBOL(osd_req_op_extent_osd_data_bvec_pos);
 
+/**
+ * osd_req_op_extent_osd_iter - Set up an operation with an iterator buffer
+ * @osd_req: The request to set up
+ * @which: Index of the operation in which to set the iter
+ * @iter: The buffer iterator
+ */
+void osd_req_op_extent_osd_iter(struct ceph_osd_request *osd_req,
+				unsigned int which, struct iov_iter *iter)
+{
+	struct ceph_osd_data *osd_data;
+
+	osd_data = osd_req_op_data(osd_req, which, extent, osd_data);
+	ceph_osd_iter_init(osd_data, iter);
+}
+EXPORT_SYMBOL(osd_req_op_extent_osd_iter);
+
 static void osd_req_op_cls_request_info_pagelist(
 			struct ceph_osd_request *osd_req,
 			unsigned int which, struct ceph_pagelist *pagelist)
···
 #endif /* CONFIG_BLOCK */
 	case CEPH_OSD_DATA_TYPE_BVECS:
 		return osd_data->bvec_pos.iter.bi_size;
+	case CEPH_OSD_DATA_TYPE_ITER:
+		return iov_iter_count(&osd_data->iter);
 	default:
 		WARN(true, "unrecognized data type %d\n", (int)osd_data->type);
 		return 0;
···
 #endif
 	} else if (osd_data->type == CEPH_OSD_DATA_TYPE_BVECS) {
 		ceph_msg_data_add_bvecs(msg, &osd_data->bvec_pos);
+	} else if (osd_data->type == CEPH_OSD_DATA_TYPE_ITER) {
+		ceph_msg_data_add_iter(msg, &osd_data->iter);
 	} else {
 		BUG_ON(osd_data->type != CEPH_OSD_DATA_TYPE_NONE);
 	}