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/tests/Makefile.am

@@ -1675,6 +1675,7 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
 	functional/mmap/mmap_seek_001_pos.ksh \
 	functional/mmap/mmap_sync_001_pos.ksh \
 	functional/mmap/mmap_write_001_pos.ksh \
+	functional/mmap/mmap_ftruncate.ksh \
 	functional/mmap/setup.ksh \
 	functional/mmp/cleanup.ksh \
 	functional/mmp/mmp_active_import.ksh \

tests/zfs-tests/tests/functional/mmap/mmap_ftruncate.ksh

@@ -0,0 +1,80 @@
+#!/bin/ksh -p
+# 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 https://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.
+#
+
+. $STF_SUITE/include/libtest.shlib
+
+#
+# This verifies that async writeback of dirty mmap()'d pages completes quickly.
+# ftruncate() is an operation that will trigger async writeback, but is not
+# itself a syncing operation, making it a useful proxy for any way the kernel
+# might trigger async writeback.
+#
+# The guts of this test is in the mmap_ftruncate program. This driver sets a
+# larger zfs_txg_timeout. Test failure occurs ftruncate() blocks waiting for
+# the writeback until the txg timeout is reached and the changes are forcibly
+# written out. Success means the DMU has accepted the changes and cleared the
+# page dirty flags.
+#
+
+TIMEOUT=180
+TESTFILE=/$TESTPOOL/truncfile
+TESTSIZE=$((2*1024*1024*1024)) # 2G
+
+verify_runnable "global"
+
+typeset claim="async writeback of dirty mmap()'d pages completes quickly"
+
+log_assert $claim
+
+log_must save_tunable TXG_TIMEOUT
+
+function cleanup
+{
+	log_must restore_tunable TXG_TIMEOUT
+	rm -f $TESTFILE
+}
+log_onexit cleanup
+
+log_must set_tunable32 TXG_TIMEOUT $TIMEOUT
+log_must zpool sync -f
+
+# run mmap_ftruncate and record the run time
+typeset -i start=$(date +%s)
+log_must mmap_ftruncate $TESTFILE $TESTSIZE
+typeset -i end=$(date +%s)
+typeset -i delta=$((end - start))
+
+# in practice, mmap_ftruncate needs a few seconds to dirty all the pages, and
+# when this test passes, the ftruncate() call itself should be near-instant.
+# when it fails, then its only the txg sync that allows ftruncate() to
+# complete, in that case, the run time will be extremely close to the timeout,
+# so to avoid any confusion at the edges, we require that it complets within
+# half the transaction time.  for any timeout higher than ~30s that should be a
+# very bright line down the middle.
+log_must test $delta -lt $((TIMEOUT / 2))
+
+log_pass $claim