Fix lseek(SEEK_DATA/SEEK_HOLE) mmap consistency

When using lseek(2) to report data/holes memory mapped regions of
the file were ignored.  This could result in incorrect results.
To handle this zfs_holey_common() was updated to asynchronously
writeback any dirty mmap(2) regions prior to reporting holes.

Additionally, while not strictly required, the dn_struct_rwlock is
now held over the dirty check to prevent the dnode structure from
changing.  This ensures that a clean dnode can't be dirtied before
the data/hole is located.  The range lock is now also taken to
ensure the call cannot race with zfs_write().

Furthermore, the code was refactored to provide a dnode_is_dirty()
helper function which checks the dnode for any dirty records to
determine its dirtiness.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #11900
Closes #12724

tests/zfs-tests/tests/functional/mmap/Makefile.am

@@ -4,7 +4,8 @@ dist_pkgdata_SCRIPTS = \
 	cleanup.ksh \
 	mmap_read_001_pos.ksh \
 	mmap_write_001_pos.ksh \
-	mmap_libaio_001_pos.ksh
+	mmap_libaio_001_pos.ksh \
+	mmap_seek_001_pos.ksh
 
 dist_pkgdata_DATA = \
 	mmap.cfg

tests/zfs-tests/tests/functional/mmap/mmap_seek_001_pos.ksh

@@ -0,0 +1,67 @@
+#!/bin/ksh -p
+#
+# 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://www.opensolaris.org/os/licensing.
+# 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) 2021 by Lawrence Livermore National Security, LLC.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/mmap/mmap.cfg
+
+#
+# DESCRIPTION:
+# lseek() data/holes for an mmap()'d file.
+#
+# STRATEGY:
+# 1. Enable compression and hole reporting for dirty files.
+# 2. Call mmap_seek binary test case for various record sizes.
+#
+
+verify_runnable "global"
+
+function cleanup
+{
+	log_must zfs set compression=off $TESTPOOL/$TESTFS
+	log_must zfs set recordsize=128k $TESTPOOL/$TESTFS
+	log_must rm -f $TESTDIR/test-mmap-file
+	log_must set_tunable64 DMU_OFFSET_NEXT_SYNC $dmu_offset_next_sync
+}
+
+log_assert "lseek() data/holes for an mmap()'d file."
+
+log_onexit cleanup
+
+# Enable hole reporting for dirty files.
+typeset dmu_offset_next_sync=$(get_tunable DMU_OFFSET_NEXT_SYNC)
+log_must set_tunable64 DMU_OFFSET_NEXT_SYNC 1
+
+# Compression must be enabled to convert zero'd blocks to holes.
+# This behavior is checked by the mmap_seek test.
+log_must zfs set compression=on $TESTPOOL/$TESTFS
+
+for bs in 4096 8192 16384 32768 65536 131072; do
+	log_must zfs set recordsize=$bs $TESTPOOL/$TESTFS
+	log_must mmap_seek $TESTDIR/test-mmap-file $((1024*1024)) $bs
+	log_must rm $TESTDIR/test-mmap-file
+done
+
+log_pass "lseek() data/holes for an mmap()'d file succeeded."