regression in DIO write behavior

[Jeff Layton <jlayton@redhat.com>]

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I've noticed a probable regression in recent kernels. When you run the
attached program on an older kernel (I used 2.6.32-642.6.2.el6.x86_64),
I see the kernel generate wsize WRITE calls on the wire.

When I run the same program on a more modern kernel (mainline as of
today), it generates a ton of page-sized I/Os instead. I've verified
that iov_iter_get_pages_alloc is returning a wsize array of pages, it
just seems like the request handling code isn't stitching them together
like it should.

Is this an expected change or a regression? I'm guessing the latter, and
that it might have crept in during the pageio rework from a couple of
years ago.

Any idea where the bug might be?
-- 
Jeff Layton <jlayton@redhat.com>

[-- Attachment #2: diotest2.c --]
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#define _GNU_SOURCE

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <sys/uio.h>
#include <unistd.h>
#include <stdlib.h>

#define	NUM_PAGES	(256)

int main(int argc, char **argv)
{
	int ret, fd;
	long pagesize;
	struct iovec	iov;

	if (argc < 2) {
		fprintf(stderr, "Usage: %s <filename>\n", argv[0]);
		return 1;
	}

	pagesize = sysconf(_SC_PAGESIZE);
	if (pagesize < 0) {
		perror("sysconf");
		return 1;
	}

	fd = open(argv[1], O_CREAT|O_WRONLY|O_DIRECT, 0666);
	if (fd < 0) {
		perror("open");
		return 1;
	}

	ret = posix_memalign(&iov.iov_base, pagesize, pagesize * NUM_PAGES);
	if (ret) {
		perror("posix_memalign");
		return 1;
	}
	iov.iov_len = pagesize * NUM_PAGES;

	ret = writev(fd, &iov, 1);
	if (ret < 0) {
		perror("writev");
		return 1;
	}

	return 0;
}