Update pin_user_pages() calls for Direct I/O

Originally #16856 updated Linux Direct I/O requests to use the new
pin_user_pages API. However, it was an oversight that this PR only
handled iov_iter's of type ITER_IOVEC and ITER_UBUF. Other iov_iter
types may try and use the pin_user_pages API if it is available. This
can lead to panics as the iov_iter is not being iterated over correctly
in zfs_uio_pin_user_pages().

Unfortunately, generic iov_iter API's that call pin_user_page_fast() are
protected as GPL only. Rather than update zfs_uio_pin_user_pages() to
account for all iov_iter types, we can simply just call
zfs_uio_get_dio_page_iov_iter() if the iov_iter type is not ITER_IOVEC
or ITER_UBUF. zfs_uio_get_dio_page_iov_iter() calls the
iov_iter_get_pages() calls that can handle any iov_iter type.

In the future it might be worth using the exposed iov_iter iterator
functions that are included in the header iov_iter.h since v6.7. These
functions allow for any iov_iter type to be iterated over and advanced
while applying a step function during iteration. This could possibly be
leveraged in zfs_uio_pin_user_pages().

A new ZFS test case was added to test that a ITER_BVEC is handled
correctly using this new code path. This test case was provided though
issue #16956.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Signed-off-by: Brian Atkinson <batkinson@lanl.gov>
Closes #16956 
Closes #17006

tests/runfiles/linux.run

@@ -103,7 +103,7 @@ tests = ['devices_001_pos', 'devices_002_neg', 'devices_003_pos']
 tags = ['functional', 'devices']
 
 [tests/functional/direct:Linux]
-tests = ['dio_write_verify']
+tests = ['dio_loopback_dev', 'dio_write_verify']
 tags = ['functional', 'direct']
 
 [tests/functional/events:Linux]

tests/zfs-tests/tests/Makefile.am

@@ -1476,6 +1476,7 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
 	functional/direct/dio_dedup.ksh \
 	functional/direct/dio_encryption.ksh \
 	functional/direct/dio_grow_block.ksh \
+	functional/direct/dio_loopback_dev.ksh \
 	functional/direct/dio_max_recordsize.ksh \
 	functional/direct/dio_mixed.ksh \
 	functional/direct/dio_mmap.ksh \

tests/zfs-tests/tests/functional/direct/dio_loopback_dev.ksh

@@ -0,0 +1,78 @@
+#!/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 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 by Triad National Security, LLC.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/direct/dio.cfg
+. $STF_SUITE/tests/functional/direct/dio.kshlib
+
+#
+# DESCRIPTION:
+# 	Verify Direct I/O reads work with loopback devices using direct=always.
+#
+# STRATEGY:
+#	1. Create raidz zpool.
+#	2. Create dataset with the direct dataset property set to always.
+#	3. Create an empty file in dataset and setup loop device on it.
+#	4. Read from loopback device.
+#
+
+verify_runnable "global"
+
+function cleanup
+{
+	if [[ -n $lofidev ]]; then
+		losetup -d $lofidev
+	fi
+	dio_cleanup
+}
+
+log_assert "Verify loopback devices with Direct I/O."
+
+if ! is_linux; then
+	log_unsupported "This is just a check for Linux Direct I/O"
+fi
+
+log_onexit cleanup
+
+# Create zpool
+log_must truncate -s $MINVDEVSIZE $DIO_VDEVS
+log_must create_pool $TESTPOOL1 "raidz" $DIO_VDEVS
+
+# Creating dataset with direct=always
+log_must eval "zfs create -o direct=always $TESTPOOL1/$TESTFS1"
+mntpt=$(get_prop mountpoint $TESTPOOL1/$TESTFS1)
+
+# Getting a loopback device
+lofidev=$(losetup -f)
+
+# Create loopback device
+log_must truncate -s 1M "$mntpt/temp_file"
+log_must losetup $lofidev "$mntpt/temp_file"
+
+# Read from looback device to make sure Direct I/O works with loopback device
+log_must dd if=$lofidev of=/dev/null count=1 bs=4k
+
+log_pass "Verified loopback devices for Direct I/O."