ZTS: mmap_ftruncate test to confirm async writeback behaviour

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17584

tests/zfs-tests/cmd/.gitignore

@@ -23,6 +23,7 @@
 /mkfiles
 /mktree
 /mmap_exec
+/mmap_ftruncate
 /mmap_libaio
 /mmap_seek
 /mmap_sync

tests/zfs-tests/cmd/Makefile.am

@@ -72,7 +72,9 @@ scripts_zfs_tests_bin_PROGRAMS += %D%/mkbusy %D%/mkfile %D%/mkfiles %D%/mktree
 %C%_mkfile_LDADD = $(LTLIBINTL)
 
 
-scripts_zfs_tests_bin_PROGRAMS += %D%/mmap_exec %D%/mmap_seek %D%/mmap_sync %D%/mmapwrite %D%/readmmap
+scripts_zfs_tests_bin_PROGRAMS += \
+	%D%/mmap_exec %D%/mmap_ftruncate %D%/mmap_seek \
+	%D%/mmap_sync %D%/mmapwrite %D%/readmmap
 %C%_mmapwrite_LDADD = -lpthread
 
 if WANT_MMAP_LIBAIO

tests/zfs-tests/cmd/mmap_ftruncate.c

@@ -0,0 +1,85 @@
+// SPDX-License-Identifier: CDDL-1.0
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://opensource.org/licenses/CDDL-1.0.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+/*
+ * Copyright (c) 2025, Klara, Inc.
+ */
+
+/*
+ * Tests async writeback behaviour. Creates a file, maps it into memory, and
+ * dirties every page within it. Then, calls ftruncate() to collapse the file
+ * back down to 0. This causes the kernel to begin writeback on the dirty
+ * pages so they can be freed, before it can complete the ftruncate() call.
+ * None of these are sync operations, so they should avoid the various "force
+ * flush" codepaths.
+ */
+
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+#define	_pdfail(f, l, s)	\
+	do { perror("[" f "#" #l "] " s); exit(2); } while (0)
+#define	pdfail(str) _pdfail(__FILE__, __LINE__, str)
+
+int
+main(int argc, char **argv) {
+	if (argc != 3) {
+		printf("usage: mmap_ftruncate <file> <size>\n");
+		exit(2);
+	}
+
+	const char *file = argv[1];
+
+	char *end;
+	off_t sz = strtoull(argv[2], &end, 0);
+	if (end == argv[2] || *end != '\0' || sz == 0) {
+		fprintf(stderr, "E: invalid size");
+		exit(2);
+	}
+
+	int fd = open(file, O_CREAT|O_TRUNC|O_RDWR, S_IRUSR|S_IWUSR);
+	if (fd < 0)
+		pdfail("open");
+
+	if (ftruncate(fd, sz) < 0)
+		pdfail("ftruncate");
+
+	char *p = mmap(NULL, sz, PROT_WRITE, MAP_SHARED, fd, 0);
+	if (p == MAP_FAILED)
+		pdfail("mmap");
+
+	for (off_t off = 0; off < sz; off += 4096)
+		p[off] = 1;
+
+	if (ftruncate(fd, 0) < 0)
+		pdfail("ftruncate");
+
+	if (munmap(p, sz) < 0)
+		pdfail("munmap");
+
+	close(fd);
+	return (0);
+}