LTP regressions due to 6dc296e7df4c ("mm: make sure all file VMAs have ->vm_ops set")

[Will Deacon <will.deacon@arm.com>]

Hi Kirill,

Your patch 6dc296e7df4c ("mm: make sure all file VMAs have ->vm_ops set")
causes some mmap regressions in LTP, which appears to use a MAP_PRIVATE
mmap of /dev/zero as a way to get anonymous pages in some of its tests
(specifically mmap10 [1]).

Dead simple reproducer below. Is this change in behaviour intentional?

Will

[1]
http://sourceforge.net/p/ltp/git/ci/1eb440c2b5fe43a3e5023015a16aa5d7d3385b1e/tree/testcases/kernel/syscalls/mmap/mmap10.c

--->8

#include <sys/mman.h>
#include <sys/stat.h>

#include <fcntl.h>
#include <stdio.h>

#define MAP_SZ	5*1024*1024

int main(void)
{
	char *foo;
	int fd;

	fd = open("/dev/zero", O_RDWR, 0666);
	if (fd < 0) {
		perror(NULL);
		return fd;
	}

	foo = mmap(NULL, MAP_SZ, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
	if (foo == MAP_FAILED) {
		perror(NULL);
		return -1;
	}

	foo[MAP_SZ >> 1] = 0; // Generates SIGBUS with 4.3-rc1
	return 0;
}

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