int ring_buffer_read_page(struct trace_buffer *buffer,
			  void **data_page, size_t len, int cpu, int full)
{
	struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
	struct ring_buffer_event *event;
	struct buffer_data_page *bpage;
	struct buffer_page *reader;
	unsigned long missed_events;
	unsigned long flags;
	unsigned int commit;
	unsigned int read;
	u64 save_timestamp;
	int ret = -1;

	if (!cpumask_test_cpu(cpu, buffer->cpumask))
		goto out;

	/*
	 * If len is not big enough to hold the page header, then
	 * we can not copy anything.
	 */
	if (len <= BUF_PAGE_HDR_SIZE)
		goto out;

	len -= BUF_PAGE_HDR_SIZE;

	if (!data_page)
		goto out;

	bpage = *data_page;
	if (!bpage)
		goto out;

	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);

	reader = rb_get_reader_page(cpu_buffer);
	if (!reader)
		goto out_unlock;

	event = rb_reader_event(cpu_buffer);

	read = reader->read;
	commit = rb_page_commit(reader);

	/* Check if any events were dropped */
	missed_events = cpu_buffer->lost_events;

	/*
	 * If this page has been partially read or
	 * if len is not big enough to read the rest of the page or
	 * a writer is still on the page, then
	 * we must copy the data from the page to the buffer.
	 * Otherwise, we can simply swap the page with the one passed in.
	 */
	if (read || (len < (commit - read)) ||
	    cpu_buffer->reader_page == cpu_buffer->commit_page) {
		struct buffer_data_page *rpage = cpu_buffer->reader_page->page;
		unsigned int rpos = read;
		unsigned int pos = 0;
		unsigned int size;

		if (full)
			goto out_unlock;

		if (len > (commit - read))
			len = (commit - read);

		/* Always keep the time extend and data together */
		size = rb_event_ts_length(event);

		if (len < size)
			goto out_unlock;

		/* save the current timestamp, since the user will need it */
		save_timestamp = cpu_buffer->read_stamp;

		/* Need to copy one event at a time */
		do {
			/* We need the size of one event, because
			 * rb_advance_reader only advances by one event,
			 * whereas rb_event_ts_length may include the size of
			 * one or two events.
			 * We have already ensured there's enough space if this
			 * is a time extend. */
			size = rb_event_length(event);
			memcpy(bpage->data + pos, rpage->data + rpos, size);

			len -= size;

			rb_advance_reader(cpu_buffer);
			rpos = reader->read;
			pos += size;

			if (rpos >= commit)
				break;

			event = rb_reader_event(cpu_buffer);
			/* Always keep the time extend and data together */
			size = rb_event_ts_length(event);
		} while (len >= size);

		/* update bpage */
		local_set(&bpage->commit, pos);
		bpage->time_stamp = save_timestamp;

		/* we copied everything to the beginning */
		read = 0;
	} else {
		/* update the entry counter */
		cpu_buffer->read += rb_page_entries(reader);
		cpu_buffer->read_bytes += BUF_PAGE_SIZE;

		/* swap the pages */
		rb_init_page(bpage);
		bpage = reader->page;
		reader->page = *data_page;
		local_set(&reader->write, 0);
		local_set(&reader->entries, 0);
		reader->read = 0;
		*data_page = bpage;

		/*
		 * Use the real_end for the data size,
		 * This gives us a chance to store the lost events
		 * on the page.
		 */
		if (reader->real_end)
			local_set(&bpage->commit, reader->real_end);
	}
	ret = read;

	cpu_buffer->lost_events = 0;

	commit = local_read(&bpage->commit);
	/*
	 * Set a flag in the commit field if we lost events
	 */
	if (missed_events) {
		/* If there is room at the end of the page to save the
		 * missed events, then record it there.
		 */
		if (BUF_PAGE_SIZE - commit >= sizeof(missed_events)) {
			memcpy(&bpage->data[commit], &missed_events,
			       sizeof(missed_events));
			local_add(RB_MISSED_STORED, &bpage->commit);
			commit += sizeof(missed_events);
		}
		local_add(RB_MISSED_EVENTS, &bpage->commit);
	}

	/*
	 * This page may be off to user land. Zero it out here.
	 */
	if (commit < BUF_PAGE_SIZE)
		memset(&bpage->data[commit], 0, BUF_PAGE_SIZE - commit);

 out_unlock:
	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);

 out:
	return ret;
}