On 10.10.24 17:30, Fabiano Rosas wrote:
Vladimir Sementsov-Ogievskiy <vsement...@yandex-team.ru> writes:
On 09.10.24 23:53, Fabiano Rosas wrote:
Vladimir Sementsov-Ogievskiy <vsement...@yandex-team.ru> writes:
On 30.09.24 17:07, Andrey Drobyshev wrote:
On 9/30/24 12:25 PM, Vladimir Sementsov-Ogievskiy wrote:
[add migration maintainers]
On 24.09.24 15:56, Andrey Drobyshev wrote:
[...]
I doubt that this a correct way to go.
As far as I understand, "inactive" actually means that "storage is not
belong to qemu, but to someone else (another qemu process for example),
and may be changed transparently". In turn this means that Qemu should
do nothing with inactive disks. So the problem is that nobody called
bdrv_activate_all on target, and we shouldn't ignore that.
Hmm, I see in process_incoming_migration_bh() we do call
bdrv_activate_all(), but only in some scenarios. May be, the condition
should be less strict here.
Why we need any condition here at all? Don't we want to activate
block-layer on target after migration anyway?
Hmm I'm not sure about the unconditional activation, since we at least
have to honor LATE_BLOCK_ACTIVATE cap if it's set (and probably delay it
in such a case). In current libvirt upstream I see such code:
/* Migration capabilities which should always be enabled as long as they
* are supported by QEMU. If the capability is supposed to be enabled on both
* sides of migration, it won't be enabled unless both sides support it.
*/
static const qemuMigrationParamsAlwaysOnItem qemuMigrationParamsAlwaysOn[] = {
{QEMU_MIGRATION_CAP_PAUSE_BEFORE_SWITCHOVER,
QEMU_MIGRATION_SOURCE},
{QEMU_MIGRATION_CAP_LATE_BLOCK_ACTIVATE,
QEMU_MIGRATION_DESTINATION},
};
which means that libvirt always wants LATE_BLOCK_ACTIVATE to be set.
The code from process_incoming_migration_bh() you're referring to:
/* If capability late_block_activate is set:
* Only fire up the block code now if we're going to restart the
* VM, else 'cont' will do it.
* This causes file locking to happen; so we don't want it to happen
* unless we really are starting the VM.
*/
if (!migrate_late_block_activate() ||
(autostart && (!global_state_received() ||
runstate_is_live(global_state_get_runstate())))) {
/* Make sure all file formats throw away their mutable metadata.
* If we get an error here, just don't restart the VM yet. */
bdrv_activate_all(&local_err);
if (local_err) {
error_report_err(local_err);
local_err = NULL;
autostart = false;
}
}
It states explicitly that we're either going to start VM right at this
point if (autostart == true), or we wait till "cont" command happens.
None of this is going to happen if we start another migration while
still being in PAUSED state. So I think it seems reasonable to take
such case into account. For instance, this patch does prevent the crash:
diff --git a/migration/migration.c b/migration/migration.c
index ae2be31557..3222f6745b 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -733,7 +733,8 @@ static void process_incoming_migration_bh(void *opaque)
*/
if (!migrate_late_block_activate() ||
(autostart && (!global_state_received() ||
- runstate_is_live(global_state_get_runstate())))) {
+ runstate_is_live(global_state_get_runstate()))) ||
+ (!autostart && global_state_get_runstate() == RUN_STATE_PAUSED)) {
/* Make sure all file formats throw away their mutable metadata.
* If we get an error here, just don't restart the VM yet. */
bdrv_activate_all(&local_err);
What are your thoughts on it?
This bug is the same as https://gitlab.com/qemu-project/qemu/-/issues/2395
Hmmm... Don't we violate "late-block-activate" contract by this?
Me go and check:
# @late-block-activate: If enabled, the destination will not activate
# block devices (and thus take locks) immediately at the end of
# migration. (since 3.0)
Yes, we'll violate it by this patch. So, for now the only exception is
when autostart is enabled, but libvirt correctly use
late-block-activate + !autostart.
Interesting, when block layer is assumed to be activated.. Aha, only in
qmp_cont().
So, what to do with this all:
Either libvirt should not use late-block-activate for migration of
stopped vm. This way target would be automatically activated
Or if libvirt still need postponed activation (I assume, for correctly
switching shared disks, etc), Libvirt should do some additional QMP
call. It can't be "cont", if we don't want to run the VM. So,
probably, we need additional "block-activate" QMP command for this.
A third option might be to unconditionally activate in qmp_migrate:
Yes. But is migration the only operation with vm which requires block
layer be activated? I think actually a lot of operation require
that.. Any block-layer releated qmp command actually. And do automatic
activation in all of them I think is a wrong way.
Yes, good point. I don't know how other commands behave in this
situation. It would be good to have an unified solution. I'll check.
Moreover, if we have explicit possibility to "postpone activation", we
should provide a way to "activate by hand".
Maybe, but it doesn't really follows. We have been activating
automatically until now, after all (from qmp_cont). Also, having to go
change libvirt code just for this is not ideal.
So I still think correct fix is reporting error from qmp_migrate when
block-layer is inactive, and add some possibility to activate through
QMP.
Unfortunately, for migration that's bad user experience: we allow the
first migration of a paused VM with no issues, then on the second one we
error out asking for a command to be run, which only does a
bdrv_activate_all() that QEMU could very well do itself.
Hmm. Now I tend to agree that it's OK to activate block-layer on "migrate"
command automatically.
Full solution should consider also another block-layer related qmp commands,
but that would require a lot more work.
--
Best regards,
Vladimir
