selftests/ublk: add hugetlbfs shmem_zc test for loop target

Add test_shmem_zc_02.sh which tests the UBLK_IO_F_SHMEM_ZC zero-copy
path on the loop target using a hugetlbfs shared buffer. Both kublk and
fio mmap the same hugetlbfs file with MAP_SHARED, sharing physical
pages. The kernel's PFN matching enables zero-copy — the loop target
reads/writes directly from the shared buffer to the backing file.

Uses standard fio --mem=mmaphuge:<path> (supported since fio 1.10),
no patched fio required.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://patch.msgid.link/20260331153207.3635125-9-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

tools/testing/selftests/ublk/Makefile

@@ -53,6 +53,7 @@ TEST_PROGS += test_part_01.sh
 TEST_PROGS += test_part_02.sh
 
 TEST_PROGS += test_shmemzc_01.sh
+TEST_PROGS += test_shmemzc_02.sh
 
 TEST_PROGS += test_stress_01.sh
 TEST_PROGS += test_stress_02.sh

tools/testing/selftests/ublk/test_shmemzc_02.sh

@@ -0,0 +1,68 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Test: shmem_zc with hugetlbfs buffer on loop target
+#
+# kublk and fio both mmap the same hugetlbfs file (MAP_SHARED),
+# so they share physical pages.  The kernel PFN match enables
+# zero-copy I/O without socket-based fd passing.
+
+. "$(cd "$(dirname "$0")" && pwd)"/test_common.sh
+
+ERR_CODE=0
+
+_prep_test "shmem_zc" "loop target hugetlbfs shmem zero-copy test"
+
+if ! _have_program fio; then
+	echo "SKIP: fio not available"
+	exit "$UBLK_SKIP_CODE"
+fi
+
+if ! grep -q hugetlbfs /proc/filesystems; then
+	echo "SKIP: hugetlbfs not supported"
+	exit "$UBLK_SKIP_CODE"
+fi
+
+# Allocate hugepages
+OLD_NR_HP=$(cat /proc/sys/vm/nr_hugepages)
+echo 10 > /proc/sys/vm/nr_hugepages
+NR_HP=$(cat /proc/sys/vm/nr_hugepages)
+if [ "$NR_HP" -lt 2 ]; then
+	echo "SKIP: cannot allocate hugepages"
+	echo "$OLD_NR_HP" > /proc/sys/vm/nr_hugepages
+	exit "$UBLK_SKIP_CODE"
+fi
+
+# Mount hugetlbfs
+HTLB_MNT=$(mktemp -d "${UBLK_TEST_DIR}/htlb_mnt_XXXXXX")
+if ! mount -t hugetlbfs none "$HTLB_MNT"; then
+	echo "SKIP: cannot mount hugetlbfs"
+	rmdir "$HTLB_MNT"
+	echo "$OLD_NR_HP" > /proc/sys/vm/nr_hugepages
+	exit "$UBLK_SKIP_CODE"
+fi
+
+HTLB_FILE="$HTLB_MNT/ublk_buf"
+fallocate -l 4M "$HTLB_FILE"
+
+_create_backfile 0 128M
+BACKFILE="${UBLK_BACKFILES[0]}"
+
+dev_id=$(_add_ublk_dev -t loop --shmem_zc --htlb "$HTLB_FILE" "$BACKFILE")
+_check_add_dev $TID $?
+
+_run_fio_verify_io --filename=/dev/ublkb"${dev_id}" \
+	--size=128M \
+	--mem=mmaphuge:"$HTLB_FILE"
+ERR_CODE=$?
+
+# Delete device first so daemon releases the htlb mmap
+_ublk_del_dev "${dev_id}"
+
+rm -f "$HTLB_FILE"
+umount "$HTLB_MNT"
+rmdir "$HTLB_MNT"
+echo "$OLD_NR_HP" > /proc/sys/vm/nr_hugepages
+
+_cleanup_test "shmem_zc"
+
+_show_result $TID $ERR_CODE