linux-kernel - LTP regressions due to 6dc296e7df4c ("mm: make sure all file VMAs have ->vm

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Message-ID: <20150914105346.GB23878@arm.com>
Date:	Mon, 14 Sep 2015 11:53:46 +0100
From:	Will Deacon <will.deacon@....com>
To:	kirill.shutemov@...ux.intel.com
Cc:	oleg@...hat.com, hpa@...or.com, luto@...capital.net,
	dave.hansen@...ux.intel.com, mingo@...e.hu, minchan@...nel.org,
	tglx@...utronix.de, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: LTP regressions due to 6dc296e7df4c ("mm: make sure all file VMAs
 have ->vm_ops set")

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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