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vmspawn-qmp: pipeline remove-fd after each blockdev-add

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QEMU keeps a monitor-side fd alive until either an explicit remove-fd
arrives or the fdset's last duplicate is closed. Today vmspawn issues
add-fd but never the matching remove-fd, so each fdset stays around for
the lifetime of the VM even after the consuming blockdev is torn down.
Pipelining a remove-fd directly after the blockdev-add that consumed
the fd hands ownership entirely to the blockdev: the fdset
auto-disposes when raw_close runs at blockdev-del time. This is the
shape needed by hotplug, where blockdev-del must clean up everything
without further coordination.

Mechanically:
- qmp_fdset_add() takes a callback/userdata pair (so callers control
  failure handling) and an optional out-param for the numeric fdset id.
  All boot-time callers keep using on_qmp_complete with a label.
- A new qmp_fdset_remove() helper sends remove-fd with caller-supplied
  callback/userdata.
- qmp_setup_ephemeral_drive captures both fdset ids and fires remove-fd
  immediately after each base/overlay file blockdev-add.
- qmp_setup_regular_drive does the same for its single file blockdev-add.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Christian Brauner
2026-04-24 14:39:25 +02:00
parent a57fbf4e6c
commit b75b6041e0
1 changed files with 62 additions and 7 deletions
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src/vmspawn/vmspawn-qmp.c
+62 -7
View File
@@ -125,9 +125,15 @@ static int on_qmp_complete(
return 0;
}
/* Send add-fd via SCM_RIGHTS; return /dev/fdset/N. Allocations run before invoke so a late
* OOM cannot orphan an fdset on QEMU's side; *ret_path is only written on full success. */
static int qmp_fdset_add(QmpClient *qmp, int fd_consume, char **ret_path) {
/* Send add-fd via SCM_RIGHTS; return /dev/fdset/N and the numeric fdset id. */
static int qmp_fdset_add(
QmpClient *qmp,
int fd_consume,
qmp_command_callback_t callback,
void *userdata,
char **ret_path,
uint64_t *ret_fdset_id) {
_cleanup_(sd_json_variant_unrefp) sd_json_variant *args = NULL;
_cleanup_close_ int fd = fd_consume;
_cleanup_free_ char *path = NULL;
@@ -136,6 +142,7 @@ static int qmp_fdset_add(QmpClient *qmp, int fd_consume, char **ret_path) {
assert(qmp);
assert(fd_consume >= 0);
assert(callback);
assert(ret_path);
id = qmp_client_next_fdset_id(qmp);
@@ -148,14 +155,39 @@ static int qmp_fdset_add(QmpClient *qmp, int fd_consume, char **ret_path) {
return -ENOMEM;
r = qmp_client_invoke(qmp, /* ret_slot= */ NULL, "add-fd", QMP_CLIENT_ARGS_FD(args, TAKE_FD(fd)),
on_qmp_complete, (void*) "add-fd");
callback, userdata);
if (r < 0)
return r;
*ret_path = TAKE_PTR(path);
if (ret_fdset_id)
*ret_fdset_id = id;
return 0;
}
/* Issue remove-fd for an fdset whose dup is now held by a blockdev. The fdset
* persists until the dup is closed (in raw_close at blockdev-del time) — see
* QEMU's monitor/fds.c:177-181 on the fds/dup_fds split. */
static int qmp_fdset_remove(
QmpClient *qmp,
uint64_t fdset_id,
qmp_command_callback_t callback,
void *userdata) {
_cleanup_(sd_json_variant_unrefp) sd_json_variant *args = NULL;
int r;
assert(qmp);
assert(callback);
r = sd_json_buildo(&args, SD_JSON_BUILD_PAIR_UNSIGNED("fdset-id", fdset_id));
if (r < 0)
return r;
return qmp_client_invoke(qmp, /* ret_slot= */ NULL, "remove-fd", QMP_CLIENT_ARGS(args),
callback, userdata);
}
typedef struct QmpFileNodeParams {
const char *node_name;
const char *filename;
@@ -399,12 +431,16 @@ static int qmp_setup_ephemeral_drive(VmspawnQmpBridge *bridge, QmpClient *qmp, D
/* Step 1-2: Pass both fds to QEMU */
_cleanup_free_ char *base_path = NULL;
r = qmp_fdset_add(qmp, TAKE_FD(drive->fd), &base_path);
uint64_t base_fdset_id;
r = qmp_fdset_add(qmp, TAKE_FD(drive->fd),
on_qmp_complete, (void*) "add-fd", &base_path, &base_fdset_id);
if (r < 0)
return log_error_errno(r, "Failed to send add-fd for base image '%s': %m", drive->path);
_cleanup_free_ char *overlay_path = NULL;
r = qmp_fdset_add(qmp, TAKE_FD(drive->overlay_fd), &overlay_path);
uint64_t overlay_fdset_id;
r = qmp_fdset_add(qmp, TAKE_FD(drive->overlay_fd),
on_qmp_complete, (void*) "add-fd", &overlay_path, &overlay_fdset_id);
if (r < 0)
return log_error_errno(r, "Failed to send add-fd for overlay of '%s': %m", drive->path);
@@ -421,6 +457,12 @@ static int qmp_setup_ephemeral_drive(VmspawnQmpBridge *bridge, QmpClient *qmp, D
if (r < 0)
return log_error_errno(r, "Failed to send blockdev-add for base file '%s': %m", drive->path);
/* The base file node now holds a dup of the fd; release the monitor's
* original so the fdset auto-frees when raw_close runs at teardown. */
r = qmp_fdset_remove(qmp, base_fdset_id, on_qmp_complete, (void*) "remove-fd");
if (r < 0)
return log_error_errno(r, "Failed to send remove-fd for base image '%s': %m", drive->path);
/* Step 4: Base image format node (read-only) */
QmpFormatNodeParams base_fmt_params = {
.node_name = base_fmt_node,
@@ -453,6 +495,11 @@ static int qmp_setup_ephemeral_drive(VmspawnQmpBridge *bridge, QmpClient *qmp, D
if (r < 0)
return log_error_errno(r, "Failed to send blockdev-add for overlay file '%s': %m", drive->path);
/* Same as for base: the overlay file node has the dup. */
r = qmp_fdset_remove(qmp, overlay_fdset_id, on_qmp_complete, (void*) "remove-fd");
if (r < 0)
return log_error_errno(r, "Failed to send remove-fd for overlay of '%s': %m", drive->path);
/* Step 6: Fire blockdev-create to format the overlay */
_cleanup_(sd_json_variant_unrefp) sd_json_variant *create_options = NULL;
r = sd_json_buildo(&create_options,
@@ -533,7 +580,9 @@ static int qmp_setup_regular_drive(VmspawnQmpBridge *bridge, QmpClient *qmp, Dri
return log_oom();
_cleanup_free_ char *fdset_path = NULL;
r = qmp_fdset_add(qmp, TAKE_FD(drive->fd), &fdset_path);
uint64_t fdset_id;
r = qmp_fdset_add(qmp, TAKE_FD(drive->fd),
on_qmp_complete, (void*) "add-fd", &fdset_path, &fdset_id);
if (r < 0)
return log_error_errno(r, "Failed to send add-fd for '%s': %m", drive->path);
@@ -549,6 +598,12 @@ static int qmp_setup_regular_drive(VmspawnQmpBridge *bridge, QmpClient *qmp, Dri
if (r < 0)
return log_error_errno(r, "Failed to send blockdev-add for '%s': %m", drive->path);
/* The file node now holds a dup of the fd; release the monitor's
* original so the fdset auto-frees when raw_close runs at teardown. */
r = qmp_fdset_remove(qmp, fdset_id, on_qmp_complete, (void*) "remove-fd");
if (r < 0)
return log_error_errno(r, "Failed to send remove-fd for '%s': %m", drive->path);
QmpFormatNodeParams fmt_params = {
.node_name = drive->qmp_node_name,
.format = drive->format,