Re: [PATCH 6/7] Introduce facilities for guest migration on top of vhost-user infrastructure

[Stefano Brivio <sbrivio@redhat.com>]

Fixed, finally. Some answers:

On Fri, 31 Jan 2025 17:14:18 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Fri, Jan 31, 2025 at 06:36:55AM +0100, Stefano Brivio wrote:
> > On Thu, 30 Jan 2025 09:32:36 +0100
> > Stefano Brivio <sbrivio@redhat.com> wrote:
> >   
> > > I would like to quickly complete the whole flow first, because I think
> > > we can inform design and implementation decisions much better at that
> > > point  
> > 
> > So, there seems to be a problem with (testing?) this. I couldn't quite
> > understand the root cause yet, and it doesn't happen with the reference
> > source.c and target.c implementations I shared.
> > 
> > Let's assume I have a connection in the source guest to 127.0.0.1:9091,
> > from 127.0.0.1:56350. After the migration, in the target, I get:
> > 
> > ---
> > socket(AF_INET, SOCK_STREAM, IPPROTO_TCP) = 79
> > setsockopt(79, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
> > bind(79, {sa_family=AF_INET, sin_port=htons(56350), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
> > sendmsg(72, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\1", iov_len=1}], msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, cmsg_data=[79]}], msg_controllen=24, msg_flags=0}, 0) = 1
> > recvfrom(72, "\1", 1, 0, NULL, NULL)    = 1
> > setsockopt(79, SOL_TCP, TCP_REPAIR_QUEUE, [2], 4) = 0
> > setsockopt(79, SOL_TCP, TCP_QUEUE_SEQ, [1788468535], 4) = 0
> > write(2, "77.6923: ", 977.6923: )                = 9
> > write(2, "Set send queue sequence for sock"..., 51Set send queue sequence for socket 79 to 1788468535) = 51
> > write(2, "\n", 1
> > )                       = 1
> > setsockopt(79, SOL_TCP, TCP_REPAIR_QUEUE, [1], 4) = 0
> > setsockopt(79, SOL_TCP, TCP_QUEUE_SEQ, [115288604], 4) = 0
> > write(2, "77.6924: ", 977.6924: )                = 9
> > write(2, "Set receive queue sequence for s"..., 53Set receive queue sequence for socket 79 to 115288604) = 53
> > write(2, "\n", 1
> > )                       = 1
> > connect(79, {sa_family=AF_INET, sin_port=htons(9091), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 EADDRNOTAVAIL (Cannot assign requested address)
> > ---
> > 
> > EADDRNOTAVAIL, according to the documentation, which seems to be
> > consistent with a glance at the implementation (that is, I must be
> > missing some issue in the kernel), should be returned on connect() if:
> > 
> >        EADDRNOTAVAIL
> >               (Internet  domain sockets) The socket referred to by
> >               sockfd had not previously been bound to  an  address
> >               and,  upon  attempting  to  bind  it to an ephemeral
> >               port, it was determined that all port numbers in the
> >               ephemeral port range are currently in use.  See  the
> >               discussion of /proc/sys/net/ipv4/ip_local_port_range
> >               in ip(7).
> > 
> > but well, of course it was bound.
> > 
> > To a port, indeed, not a full address, that is, any (0.0.0.0) and
> > address port, but I think for the purposes of this description that
> > bind() call is enough.  
> 
> So, I was wondering if binding to 0.0.0.0 is sufficient for a repaired
> socket.

It is.

> Usually, of course, that 0.0.0.0 would be resolved to a real
> address at connect() time.  But TCP_REPAIR's version of connect()
> bypasses a bunch of the usual connect logic, so maybe we need an
> explicit address here.

No need.

> ...but that doesn't explain the difference between passt and your test
> implementation.

The difference that actually matters is that the test implementation
terminates, and that has the equivalent effect of switching off repair
mode for the closed sockets, which frees up all the associated context,
including the port.

Usually, there are no valid operations on closed sockets (not even
close()). This is the first exception I ever met: you can set
TCP_REPAIR_OFF.

But there's a catch: you can't pass a closed socket in repair mode via
SCM_RIGHTS (well, I'm fairly sure nobody approached this level of
insanity before): you get EBADF (which is an understatement).

And there's another catch: if you actually try to do that, even if it
fails, that has the same effect of clearing the socket entirely: you
free up the port.

But we can't use this, unfortunately, because if we do, the peer will
get a zero-length read (EOF). Now, I could reintroduce a "quit" command
in passt-repair, and we would know that EOF doesn't actually mean
completion, but it complicates things again.

What works, though, is simply terminating. We can't do that before
VHOST_USER_CHECK_DEVICE_STATE, but just after that. That's what I
implemented at the moment (updated patches coming soon).

> > Is this related to SO_REUSEADDR? I need it (on both source and target)
> > because, at least in my tests, source and target are on the same
> > machine, in the same namespace. If I drop it:  
> 
> Again, I can think of various problems that not having the same
> address available on source and dest might have, but not any which
> explain the difference between passt and the experimental impl.
> 
> > ---
> > bind(79, {sa_family=AF_INET, sin_port=htons(46280), sin_addr=inet_addr("0.0.0.0")}, 16) = -1 EADDRINUSE (Address already in use)
> > ---
> > 
> > as expected.
> > 
> > However, in my reference implementation, with a connection from
> > 127.0.0.1:9998 to 127.0.0.1:9091, this is what the target does:
> > 
> > ---
> > socket(AF_INET, SOCK_STREAM, IPPROTO_TCP) = 3
> > setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
> > bind(3, {sa_family=AF_INET, sin_port=htons(9998), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
> > socket(AF_UNIX, SOCK_STREAM, 0)         = 4
> > unlink("/tmp/repair.sock")              = 0
> > bind(4, {sa_family=AF_UNIX, sun_path="/tmp/repair.sock"}, 110) = 0
> > listen(4, 1)                            = 0
> > accept(4, NULL, NULL)                   = 5
> > sendmsg(5, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\1", iov_len=1}], msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, cmsg_data=[3]}], msg_controllen=24, msg_flags=0}, 0) = 1
> > recvfrom(5, "\1", 1, 0, NULL, NULL)     = 1
> > setsockopt(3, SOL_TCP, TCP_REPAIR_QUEUE, [2], 4) = 0
> > setsockopt(3, SOL_TCP, TCP_QUEUE_SEQ, [1612504019], 4) = 0
> > setsockopt(3, SOL_TCP, TCP_REPAIR_QUEUE, [1], 4) = 0
> > setsockopt(3, SOL_TCP, TCP_QUEUE_SEQ, [1756508956], 4) = 0
> > connect(3, {sa_family=AF_INET, sin_port=htons(9091), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
> > ---
> > 
> > The only obvious difference is that, here, I'm not binding to an
> > ephemeral port: the source port (in both source and target "guests") is
> > 9998.
> > 
> > Fine, so I tried forcing a lower port in passt (source) as well, and
> > this is what I get in the target now:
> > 
> > ---
> > socket(AF_INET, SOCK_STREAM, IPPROTO_TCP) = 79
> > setsockopt(79, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
> > bind(79, {sa_family=AF_INET, sin_port=htons(9000), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
> > sendmsg(72, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\1", iov_len=1}], msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, cmsg_data=[79]}], msg_controllen=24, msg_flags=0}, 0) = 1
> > recvfrom(72, "\1", 1, 0, NULL, NULL)    = 1
> > setsockopt(79, SOL_TCP, TCP_REPAIR_QUEUE, [2], 4) = 0
> > setsockopt(79, SOL_TCP, TCP_QUEUE_SEQ, [-348109334], 4) = 0
> > write(2, "46.9751: ", 946.9751: )                = 9
> > write(2, "Set send queue sequence for sock"..., 51Set send queue sequence for socket 79 to 3946857962) = 51
> > write(2, "\n", 1
> > )                       = 1
> > setsockopt(79, SOL_TCP, TCP_REPAIR_QUEUE, [1], 4) = 0
> > setsockopt(79, SOL_TCP, TCP_QUEUE_SEQ, [-1820322671], 4) = 0
> > write(2, "46.9752: ", 946.9752: )                = 9
> > write(2, "Set receive queue sequence for s"..., 54Set receive queue sequence for socket 79 to 2474644625) = 54
> > write(2, "\n", 1
> > )                       = 1
> > connect(79, {sa_family=AF_INET, sin_port=htons(9091), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 EADDRNOTAVAIL (Cannot assign requested address)
> > ---
> > 
> > no obvious difference. I'll try binding to an explicit address, next,
> > but I have no idea why 1. we get EADDRNOTAVAIL after a bind() and 2. it
> > works with the reference implementation.  
> 
> I have no ideas yet :(.
> 
> > Yes, I explicitly close() the socket in the source passt now, but that
> > doesn't change things.
> > 
> > This is presumably just an issue with testing, because in real use
> > cases source and target guests would be on different machines. Another
> > idea could be separating the namespaces.  
> 
> Well, if that's relevant to the problem which isn't clear yet.  I
> mean, I guess it's worth trying with source and dest in different
> namespaces.
> 
> > I can't just run source and target passt in two instances of pasta
> > --config-net, because pasta would run into the same issue,  
> 
> Uh.. which same issue?  pasta's not trying to do any TCP_REPAIR stuff
> or migration.

Same issue in the sense that if I connect namespaces with pasta, I
can't migrate a connection between them, because pasta can't migrate a
connection. It would close it and try to reopen it.

> > but I could
> > isolate one namespace with it, then add two network namespaces inside
> > that, and connect them with veth pairs.  
> 
> Two pasta instances actually sounds like a better bet to me, because
> the two "hosts" will have the same address, which is what we'd expect
> for a "real" migration - and it kind of has to be the case for the
> host side connections to work afterwards.

Eh, yes, but we're back to the original problem. A veth interface
wouldn't care, instead.

Anyway, no need, it's finally working now.

-- 
Stefano