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Subject: [PULL v2 02/15] qemu-nbd: Support -s and -l at the same time Date: Mon, 12 Oct 2026 16:29:06 -0500 Message-ID: <20261012212945.84275-19-eblake@redhat.com> In-Reply-To: <20261012212945.84275-17-eblake@redhat.com> References: <20261012212945.84275-17-eblake@redhat.com> MIME-Version: 1.0 Content-type: text/plain Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 Received-SPF: pass client-ip=170.10.129.124; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 10 X-Spam_score: 1.0 X-Spam_bar: + X-Spam_report: (1.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.24, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org qemu-nbd already has the ability to wrap a read-write layer on top of the export of a read-only image (-s) and the ability to export an internal snapshot rather than the main image in a qcow2 file (-l SNAPID). But it gave a confusing error message when trying to mix the two, because the snapshot overlay created first no longer has access to the internal snapshots of the image being wrapped. WIth a bit of finesse, this useful scenario is now supported. https://gitlab.com/qemu-project/qemu/-/work_items/159 Signed-off-by: Eric Blake Message-ID: <20260408021636.794794-2-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy [eblake: improve test with Vladimir's suggestions, add lock to satisfy TSAN] Signed-off-by: Eric Blake --- docs/tools/qemu-nbd.rst | 7 +- qemu-nbd.c | 22 ++++- tests/qemu-iotests/tests/qemu-nbd-snapshots | 81 +++++++++++++++++++ .../qemu-iotests/tests/qemu-nbd-snapshots.out | 39 +++++++++ 4 files changed, 143 insertions(+), 6 deletions(-) create mode 100755 tests/qemu-iotests/tests/qemu-nbd-snapshots create mode 100644 tests/qemu-iotests/tests/qemu-nbd-snapshots.out diff --git a/docs/tools/qemu-nbd.rst b/docs/tools/qemu-nbd.rst index f82ea5fd77b..336b4aa46e7 100644 --- a/docs/tools/qemu-nbd.rst +++ b/docs/tools/qemu-nbd.rst @@ -95,9 +95,10 @@ driver options if :option:`--image-opts` is specified. .. option:: -l, --load-snapshot=SNAPSHOT_PARAM - Load an internal snapshot inside *filename* and export it - as an read-only device, SNAPSHOT_PARAM format is - ``snapshot.id=[ID],snapshot.name=[NAME]`` or ``[ID_OR_NAME]`` + Load an internal snapshot inside *filename* and export it. + SNAPSHOT_PARAM format is ``snapshot.id=[ID],snapshot.name=[NAME]`` + or ``[ID_OR_NAME]``. The export is read-only unless ``-s`` is also + in use. .. option:: --cache=CACHE diff --git a/qemu-nbd.c b/qemu-nbd.c index ed5895861bb..fcca88932a3 100644 --- a/qemu-nbd.c +++ b/qemu-nbd.c @@ -534,6 +534,7 @@ int main(int argc, char **argv) { BlockBackend *blk; BlockDriverState *bs; + BlockDriverState *orig_bs; uint64_t dev_offset = 0; bool readonly = false; bool disconnect = false; @@ -716,7 +717,7 @@ int main(int argc, char **argv) } else { sn_id_or_name = optarg; } - /* fall through */ + break; case 'r': readonly = true; flags &= ~BDRV_O_RDWR; @@ -1110,6 +1111,15 @@ int main(int argc, char **argv) bdrv_init(); atexit(qemu_nbd_shutdown); + /* + * -s and -l can be used together to create read-write wrapper + * around an internal snapshot. Otherwise, -l in isolation implies + * read-only. + */ + if ((sn_opts || sn_id_or_name) && (flags & BDRV_O_SNAPSHOT) == 0) { + readonly = true; + flags &= ~BDRV_O_RDWR; + } opts.srcpath = argv[optind]; if (imageOpts) { QemuOpts *o; @@ -1155,13 +1165,19 @@ int main(int argc, char **argv) blk_set_enable_write_cache(blk, !writethrough); + orig_bs = bs; + if (flags & BDRV_O_SNAPSHOT) { + WITH_GRAPH_RDLOCK_GUARD() { + orig_bs = bs->backing->bs; + } + } if (sn_opts) { - ret = bdrv_snapshot_load_tmp(bs, + ret = bdrv_snapshot_load_tmp(orig_bs, qemu_opt_get(sn_opts, SNAPSHOT_OPT_ID), qemu_opt_get(sn_opts, SNAPSHOT_OPT_NAME), &local_err); } else if (sn_id_or_name) { - ret = bdrv_snapshot_load_tmp_by_id_or_name(bs, sn_id_or_name, + ret = bdrv_snapshot_load_tmp_by_id_or_name(orig_bs, sn_id_or_name, &local_err); } if (ret < 0) { diff --git a/tests/qemu-iotests/tests/qemu-nbd-snapshots b/tests/qemu-iotests/tests/qemu-nbd-snapshots new file mode 100755 index 00000000000..ce62e97877b --- /dev/null +++ b/tests/qemu-iotests/tests/qemu-nbd-snapshots @@ -0,0 +1,81 @@ +#!/usr/bin/env bash +# group: rw quick +# +# Test qemu-nbd snapshot handling +# +# Copyright (C) Red Hat, Inc. +# +# SPDX-License-Identifier: GPL-2.0-or-later + +seq="$(basename $0)" +echo "QA output created by $seq" + +status=1 # failure is the default! + +_cleanup() +{ + _cleanup_test_img + nbd_server_stop +} +trap "_cleanup; exit \$status" 0 1 2 3 15 + +# get standard environment, filters and checks +cd .. +. ./common.rc +. ./common.filter +. ./common.nbd + +_supported_fmt qcow2 +_supported_proto file +_supported_os Linux +_require_command QEMU_NBD + +echo +echo "=== Initial image setup ===" +echo + +# Create qcow2 image with one internal snapshot +_make_test_img 10M +$QEMU_IO -c 'w -P 1 3M 1M' -f $IMGFMT "$TEST_IMG" | _filter_qemu_io +$QEMU_IMG snapshot -c snap -f $IMGFMT "$TEST_IMG" | _filter_qemu_img +$QEMU_IO -c 'w -P 2 3M 1M' -f $IMGFMT "$TEST_IMG" | _filter_qemu_io + +echo +echo "=== Main image in snapshot mode ===" +echo + +nbd_server_start_unix_socket -s -f qcow2 "$TEST_IMG" +$QEMU_IO -c 'r -P 2 3M 1M' -c 'w -P 3 3M 1M' -c 'r -P 3 3M 1M' -f raw \ + nbd:unix:$nbd_unix_socket | _filter_qemu_io +nbd_server_stop + +echo +echo "=== Read-only snapshot access ===" +echo + +nbd_server_start_unix_socket -l snap -f qcow2 "$TEST_IMG" +$QEMU_IO -r -c 'r -P 1 3M 1M' -f raw \ + nbd:unix:$nbd_unix_socket | _filter_qemu_io +$QEMU_IO -c 'r -P 1 3M 1M' -f raw \ + nbd:unix:$nbd_unix_socket 2>&1 | _filter_qemu_io | _filter_testdir +nbd_server_stop + +echo +echo "=== Combine -s and -l for read-write internal snapshot access ===" +echo + +nbd_server_start_unix_socket -s -l snap -f qcow2 "$TEST_IMG" +$QEMU_IO -c 'r -P 1 3M 1M' -c 'w -P 3 3M 1M' -c 'r -P 3 3M 1M' -f raw \ + nbd:unix:$nbd_unix_socket | _filter_qemu_io +nbd_server_stop + +echo +echo "=== Check that original image is unchanged ===" +echo + +$QEMU_IO -c 'r -P 2 3M 1M' -f $IMGFMT "$TEST_IMG" | _filter_qemu_io + +# success, all done +echo '*** done' +rm -f $seq.full +status=0 diff --git a/tests/qemu-iotests/tests/qemu-nbd-snapshots.out b/tests/qemu-iotests/tests/qemu-nbd-snapshots.out new file mode 100644 index 00000000000..47289c30f1d --- /dev/null +++ b/tests/qemu-iotests/tests/qemu-nbd-snapshots.out @@ -0,0 +1,39 @@ +QA output created by qemu-nbd-snapshots + +=== Initial image setup === + +Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=10485760 +wrote 1048576/1048576 bytes at offset 3145728 +1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) +wrote 1048576/1048576 bytes at offset 3145728 +1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) + +=== Main image in snapshot mode === + +read 1048576/1048576 bytes at offset 3145728 +1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) +wrote 1048576/1048576 bytes at offset 3145728 +1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) +read 1048576/1048576 bytes at offset 3145728 +1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) + +=== Read-only snapshot access === + +read 1048576/1048576 bytes at offset 3145728 +1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) +qemu-io: can't open device nbd:unix:SOCK_DIR/qemu-nbd.sock: Could not open image: Permission denied + +=== Combine -s and -l for read-write internal snapshot access === + +read 1048576/1048576 bytes at offset 3145728 +1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) +wrote 1048576/1048576 bytes at offset 3145728 +1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) +read 1048576/1048576 bytes at offset 3145728 +1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) + +=== Check that original image is unchanged === + +read 1048576/1048576 bytes at offset 3145728 +1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) +*** done -- 2.56.0