generic: add regression test for stale mmap reads

This adds a regression test for the following kernel patch:

  dax: fix data corruption due to stale mmap reads

The above patch fixes an issue where users of DAX can suffer data
corruption from stale mmap reads via the following sequence:

- open an mmap over a 2MiB hole

- read from a 2MiB hole, faulting in a 2MiB zero page

- write to the hole with write(3p).  The write succeeds but we incorrectly
  leave the 2MiB zero page mapping intact.

- via the mmap, read the data that was just written.  Since the zero page
  mapping is still intact we read back zeroes instead of the new data.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

.gitignore

@@ -147,6 +147,7 @@
 /src/renameat2
 /src/t_rename_overwrite
 /src/t_mmap_dio
+/src/t_mmap_stale_pmd
 
 # dmapi/ binaries
 /dmapi/src/common/cmd/read_invis

src/Makefile

@@ -12,7 +12,7 @@ TARGETS = dirstress fill fill2 getpagesize holes lstat64 \
 	godown resvtest writemod makeextents itrash rename \
 	multi_open_unlink dmiperf unwritten_sync genhashnames t_holes \
 	t_mmap_writev t_truncate_cmtime dirhash_collide t_rename_overwrite \
-	holetest t_truncate_self t_mmap_dio af_unix
+	holetest t_truncate_self t_mmap_dio af_unix t_mmap_stale_pmd
 
 LINUX_TARGETS = xfsctl bstat t_mtab getdevicesize preallo_rw_pattern_reader \
 	preallo_rw_pattern_writer ftrunc trunc fs_perms testx looptest \

src/t_mmap_stale_pmd.c

@@ -0,0 +1,73 @@
+#include <errno.h>
+#include <fcntl.h>
+#include <libgen.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#define MiB(a) ((a)*1024*1024)
+
+void err_exit(char *op)
+{
+	fprintf(stderr, "%s: %s\n", op, strerror(errno));
+	exit(1);
+}
+
+int main(int argc, char *argv[])
+{
+	volatile int a __attribute__((__unused__));
+	char *buffer = "HELLO WORLD!";
+	char *data;
+	int fd, err, ret = 0;
+
+	if (argc < 2) {
+		printf("Usage: %s <pmem file>\n", basename(argv[0]));
+		exit(0);
+	}
+
+	fd = open(argv[1], O_RDWR|O_CREAT, S_IRUSR|S_IWUSR);
+	if (fd < 0)
+		err_exit("fd");
+
+	/*
+	 * This allows us to map a huge zero page, and we do it at a non-zero
+	 * offset for a little additional testing.
+	 */
+	ftruncate(fd, 0);
+	ftruncate(fd, MiB(4));
+
+	data = mmap(NULL, MiB(2), PROT_READ, MAP_SHARED, fd, MiB(2));
+
+	/*
+	 * This faults in a 2MiB zero page to satisfy the read.
+	 * 'a' is volatile so this read doesn't get optimized out.
+	 */
+	a = data[0];
+
+	pwrite(fd, buffer, strlen(buffer), MiB(2));
+
+	/*
+	 * Try and use the mmap to read back the data we just wrote with
+	 * pwrite().  If the kernel bug is present the mapping from the 2MiB
+	 * zero page will still be intact, and we'll read back zeros instead.
+	 */
+	if (strncmp(buffer, data, strlen(buffer))) {
+		fprintf(stderr, "strncmp mismatch: '%s' vs '%s'\n", buffer,
+				data);
+		ret = 1;
+	}
+
+	err = munmap(data, MiB(2));
+	if (err < 0)
+		err_exit("munmap");
+
+	err = close(fd);
+	if (err < 0)
+		err_exit("close");
+
+	return ret;
+}

tests/generic/428

@@ -0,0 +1,60 @@
+#! /bin/bash
+# FS QA Test 428
+#
+# This is a regression test for kernel patch:
+#  dax: fix data corruption due to stale mmap reads
+# created by Ross Zwisler <ross.zwisler@linux.intel.com>
+#
+#-----------------------------------------------------------------------
+# Copyright (c) 2017 Intel Corporation.  All Rights Reserved.
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it would be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write the Free Software Foundation,
+# Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+#-----------------------------------------------------------------------
+#
+
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+
+here=`pwd`
+tmp=/tmp/$$
+status=1	# failure is the default!
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+_cleanup()
+{
+	cd /
+	rm -f $tmp.*
+}
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+
+# remove previous $seqres.full before test
+rm -f $seqres.full
+
+# Modify as appropriate.
+_supported_fs generic
+_supported_os Linux
+_require_test
+_require_test_program "t_mmap_stale_pmd"
+
+# real QA test starts here
+src/t_mmap_stale_pmd $TEST_DIR/testfile
+
+# success, all done
+echo "Silence is golden"
+status=0
+exit

tests/generic/428.out

@@ -0,0 +1,2 @@
+QA output created by 428
+Silence is golden

tests/generic/group

@@ -430,3 +430,4 @@
 425 auto quick attr
 426 auto quick exportfs
 427 auto quick aio rw
+428 auto quick