On Wed, 26 Feb 2025 11:27:32 +1100 David Gibson <david(a)gibson.dropbear.id.au> wrote:On Tue, Feb 25, 2025 at 06:43:16PM +0100, Stefano Brivio wrote:("-incoming"), yes, see src/qemu/qemu_migration.c, qemuMigrationDstPrepare().On Tue, 25 Feb 2025 16:51:30 +1100 David Gibson <david(a)gibson.dropbear.id.au> wrote:[snip] Thanks for the detailed instructions. More complex than I might have liked, but oh well.From Red Hat internal testing we've had some reports that if attempting to migrate without passt-repair, the failure mode is uglier than we'd like. The migration fails, which is somewhat expected, but we don't correctly roll things back on the source, so it breaks network there as well. Handle this more gracefully allowing the migration to proceed in this case, but allow TCP connections to break I've now tested this reasonably: * I get a clean migration if there are now active flows * Migration completes, although connections are broken if passt-repair isn't connected * Basic test suite (minus perf) I didn't manage to test with libvirt yet, but I'm pretty convinced the behaviour should be better than it was.I did, and it is. The series looks good to me and I would apply it as it is, but I'm waiting a bit longer in case you want to try out some variations based on my tests as well. Here's what I did.$ virsh migrate --verbose --p2p --live --unsafe alpine --tunneled qemu+ssh://88.198.0.161:10951/session Migration: [97.59 %]error: End of file while reading data: : Input/output error ...despite --verbose the error doesn't tell much (perhaps I need LIBVIRT_DEBUG=1 instead?), but passt terminates at this point. With this series (I just used 'make install' from the local build), migration succeeds instead: $ virsh migrate --verbose --p2p --live --unsafe alpine --tunneled qemu+ssh://88.198.0.161:10951/session Migration: [100.00 %] Now, on the target, I still have to figure out how to tell libvirt to start QEMU and prepare for the migration (equivalent of '-incoming' as we use in our tests), instead of just starting a new instance like it does. Otherwise, I have no chance to start passt-repair there. Perhaps it has something to do with persistent mode described here:Ah. So I'm pretty sure virsh migrate will automatically start qemu with --incoming on the target.IIUC the problem here is more about timing: we want it to start it early, so that we have a chance to start passt-repair and let it connect before the migration actually happens.For the timing itself, we could actually wait for passt-repair to be there, with a timeout (say, 100ms). We could also modify passt-repair to set up an inotify watcher if the socket isn't there yet.Crud... I didn't think of this before. I don't know that there's any sensible way to do this without having libvirt managing passt-repair as well.But we can't really use it as we're assuming that passt-repair will run with capabilities virtqemud doesn't want/need.I mean it's not impossible there's some option to do this, but I doubt there's been any reason before for something outside of libvirt to control the timing of the target qemu's creation. I think we need to ask libvirt people about this.I'm looking into it (and perhaps virtiofsd had similar troubles?).Yeah.. I don't think this is relevant.Well, "check", yes, but I'm not even setting an error code. I haven't tried your 3/3 yet but look at "(null)" resulting from: flow_err(flow, "Can't set up socket: %s, drop", strerror_(rc)); ...rc is 0.and --listen-address, but I'm not quite sure yet. That is, I could only test different failures (early one on source, or later one on target) with this, not a complete successful migration.Ah, yes, that is a higher priority fragile case.There are more fragile cases that I'm looking to fix, particularly the die()s in flow_migrate_source_rollback() and elsewhere, however I ran into various complications that I didn't manage to sort out today. I'll continue looking at those tomorrow. I'm now pretty confident that those additional fixes won't entirely supersede the changes in this series, so it should be fine to apply these on their own.By the way, I think the somewhat less fragile/more obvious case where we fail clumsily is when the target doesn't have the same address as the source (among other possible addresses). In that case, we fail (and terminate) with a rather awkward:93.7217: ERROR: Failed to bind socket for migrated flow: Cannot assign requested address 93.7218: ERROR: Flow 0 (TCP connection): Can't set up socket: (null), drop 93.7331: ERROR: Selecting TCP_SEND_QUEUE, socket 1: Socket operation on non-socket 93.7333: ERROR: Unexpected reply from TCP_REPAIR helper: -100 that's because, oops, I only took care of socket() failures in tcp_flow_repair_socket(), but not bind() failures (!). Sorry.No, you check for errors on both.The problem is that in tcp_flow_migrate_target() we cancel the flow allocation and carry on - but the source will still send information for this flow, putting us out of sync with the stream.That, too, yes.I think it's great that you could (practically) solve it with three lines...Once that's fixed, flow_migrate_target() should also take care of decreasing 'count' accordingly. I just had a glimpse but didn't really try to sketch a fix.Adjusting count won't do the job. Instead we'd need to keep the flow around, but marked as "dead" somehow, so that we read but discard the incoming information for it. The MIGRATING state I added in one of my drafts was supposed to help with this sort of thing. But that's quite a complex change.Hrm... at least in the near term, I think it might actually be easier to set IP_FREEBIND when we create sockets for in-migrating flows. That way we can process them normally, they just won't do much without the address set. It has the additional advantage that it should work if the higher layers only move the IP just after the migration, instead of in advance.Perhaps we want it anyway, but I wonder: what happens if we turn repair mode off and we bound to a non-local address? I suppose we won't send out anything, but I'm not sure. If we send out the first keep-alive segment with a wrong address, we probably ruined the connection. Once I find a solution for the target libvirt/passt-repair thing (and the remaining SELinux issues), I'll try to have a look at this too. I haven't tried yet a migration with a mismatching address on the target and passt-repair available. -- Stefano