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From: | priyankar jain |
Subject: | Re: [RFC] dbus-vmstate: Connect to the dbus only during the migration phase |
Date: | Wed, 2 Dec 2020 22:33:01 +0530 |
User-agent: | Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 |
On 02/12/20 9:46 pm, Daniel P. Berrangé wrote:
On Wed, Dec 02, 2020 at 09:25:27PM +0530, priyankar jain wrote:On 20/11/20 12:17 am, Daniel P. Berrangé wrote:On Thu, Nov 19, 2020 at 06:28:55PM +0000, Priyankar Jain wrote:Today, dbus-vmstate maintains a constant connection to the dbus. This is problematic for a number of reasons: 1. If dbus-vmstate is attached during power-on, then the device holds the unused connection for a long period of time until migration is triggered, thus unnecessarily occupying dbus. 2. Similarly, if the dbus is restarted in the time period between VM power-on (dbus-vmstate initialisation) and migration, then the migration will fail. The only way to recover would be by re-initialising the dbus-vmstate object. 3. If dbus is not available during VM power-on, then currently dbus-vmstate initialisation fails, causing power-on to fail. 4. For a system with large number of VMs, having multiple QEMUs connected to the same dbus can lead to a DoS for new connections.The expectation is that there is a *separate* dbus daemon created for each QEMU instance. There should never be multiple QEMUs connected to the same dbus instance, nor should it ever connect to the common dbus instances provided by most Linux distros. None of these 4 issues should apply when each QEMU has its own dedicated dbus instance AFAICT. Regards, DanielHow does having a separate dbus daemon resolve issue (2)? If any daemon restarts between VM power-on and migration, the dbus-vmstate object for that VM would have to be reinitialized, no?The private dbus damon for QEMU is expected to exist for the lifetime of that QEMU process.
Totally agree on the expectation. But any external stimuli (maybe unintended) can easily break this condition, and this would indeed result into a situation where the VM is basically non migratable until the VM is powered cycled or the dbus-vmstate is removed by manual intervention.
Secondly, having dbus-vmstate backend connect to dbus at migration time eases the complexity for any management plane to recover in these failure situations by monitoring dbus and restarting it with the same params if dbus gets killed without affecting QEMU.
Secondly, on a setup with large number of VMs, having separate dbus-daemons leads to high cummulative memory usage by dbus daemons, is it a feasible approach to spawn a new dbus-daemon for every QEMU, given the fact that majority of the security aspect lies with the dbus peers, apart from the SELinux checks provided by dbus.The memory usage of a dbus daemon shouldn't be that high. A large portion of the memory footprint should be readony pages shared between all dbus procsses. The private usage should be a functional of number of clients and the message traffic. Do you have any measured figures you're concerned with ?
One of our setup had a long running private dbus-daemon (nearly 4-5 days) in the destination hypervisor after performing migration, it was showing the following memory usage (figures in kB):
Virt - 90980 Rss - 19576 Total - 110556Extrapolating these figures for 100s of daemons results in considerable Rss usage. These figures were taken using `top` linux utility. But I had not considered the readonly shared pages aspect at the time of capture.
Regards, Priyankar
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