[linux-next] splice call weird results

[Cyrill Gorcunov <gorcunov@gmail.com>]

Hi! While been trying to run criu on linux-next (due to recent Kirill's patch related
to /proc/pid/clear_refs with THP enabled) I noticed that it fails dumping programs when
moves data from memory pages into an image file. So I wrote pretty idiotic test and
run it on current fedora 20 kernel and then on linux-next.

fedora-20 output
----------------
[cyrill@moon criu] ~/pipe 
Opened pipe-test.SQBcJa
vmspliced 8192 bytes
spliced 4096 bytes

linux-next
----------
[root@fc criu]# ~/pipe
Opened pipe-test.9nZSW7
vmspliced 8192 bytes
spliced 8192 bytes

In test I fill pipe with 8K data then splice 4K of it into a file (the test
is below). Is it intended, or I did some silly mistake?
---
#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>

#include <sys/uio.h>

#define ARRAY_SIZE(x)	(sizeof(x)/sizeof(x[0]))

static char buf[8 << 20];

int main(int argc, char *argv[])
{
	char *tmpname, t[64] = "pipe-test.XXXXXX";
	int _pipe[2], fd, ret = 0;

	struct iovec iov[] = {
		[0] = {
			.iov_base	= &buf[0],
			.iov_len	= 4096,
		},

		[1] = {
			.iov_base	= &buf[4096],
			.iov_len	= 4096,
		},
	};

	if (pipe(_pipe)) {
		perror("Can't create pipe");
		exit(1);
	}

	memset(buf, 0x1, sizeof(buf));
	tmpname = mktemp(t);

	fd = open(tmpname, O_RDWR | O_CREAT | O_TRUNC);
	if (fd < 0) {
		perror("Can't open temp file");
		close(_pipe[0]);
		close(_pipe[1]);
		exit(1);
	}
	printf("Opened %s\n", tmpname);

	ret = vmsplice(_pipe[1], iov, ARRAY_SIZE(iov), SPLICE_F_GIFT);
	printf("vmspliced %li bytes\n", (long)ret);

	ret = splice(_pipe[0], NULL, fd, NULL, iov[0].iov_len, SPLICE_F_MOVE);
	printf("spliced %li bytes\n", (long)ret);

out:
	close(_pipe[0]);
	close(_pipe[1]);
	close(fd);
	unlink(tmpname);
	return ret;
}