checkpoint: support lazy migration

With the help of userfaultfd CRIU supports lazy migration. Lazy
migration means that memory pages are only transferred from the
migration source to the migration destination on page fault.

This enables to reduce the downtime during process or container
migration to a minimum as the memory does not need to be transferred
during migration.

Lazy migration currently depends on userfaultfd being available on the
current Linux kernel and if the used CRIU version supports lazy
migration. Both dependencies can be checked by querying CRIU via RPC if
the lazy migration feature is available. Using feature checking instead
of version comparison enables runC to use CRIU features from the
criu-dev branch. This way the user can decide if lazy migration should
be available by choosing the right kernel and CRIU branch.

To use lazy migration the CRIU process during dump needs to dump
everything besides the memory pages and then it opens a network port
waiting for remote page fault requests:

 # runc checkpoint httpd --lazy-pages --page-server 0.0.0.0:27 \
  --status-fd /tmp/postcopy-pipe

In this example CRIU will hang/wait once it has opened the network port
and wait for network connection. As runC waits for CRIU to finish it
will also hang until the lazy migration has finished. To know when the
restore on the destination side can start the '--status-fd' parameter is
used:

 #️ runc checkpoint --help | grep status
  --status-fd value   criu writes \0 to this FD once lazy-pages is ready

The parameter '--status-fd' is directly from CRIU and this way the
process outside of runC which controls the migration knows exactly when
to transfer the checkpoint (without memory pages) to the destination and
that the restore can be started.

On the destination side it is necessary to start CRIU in 'lazy-pages'
mode like this:

 # criu lazy-pages --page-server --address 192.168.122.3 --port 27 \
  -D checkpoint

and tell runC to do a lazy restore:

 # runc restore -d --image-path checkpoint --work-path checkpoint \
  --lazy-pages httpd

If both processes on the restore side have the same working directory
'criu lazy-pages' creates a unix domain socket where it waits for
requests from the actual restore. runC starts CRIU restore in lazy
restore mode and talks to 'criu lazy-pages' that it wants to restore
memory pages on demand. CRIU continues to restore the process and once
the process is running and accesses the first non-existing memory page
the 'criu lazy-pages' server will request the page from the source
system. Thus all pages from the source system will be transferred to the
destination system. Once all pages have been transferred runC on the
source system will end and the container will have finished migration.

This can also be combined with CRIU's pre-copy support. The combination
of pre-copy and post-copy (lazy migration) provides the possibility to
migrate containers with minimal downtimes.

Some additional background about post-copy migration can be found in
these articles:

 https://lisas.de/~adrian/?p=1253
 https://lisas.de/~adrian/?p=1183

Signed-off-by: Adrian Reber <areber@redhat.com>

checkpoint.go

@@ -30,6 +30,8 @@ checkpointed.`,
 		cli.BoolFlag{Name: "tcp-established", Usage: "allow open tcp connections"},
 		cli.BoolFlag{Name: "ext-unix-sk", Usage: "allow external unix sockets"},
 		cli.BoolFlag{Name: "shell-job", Usage: "allow shell jobs"},
+		cli.BoolFlag{Name: "lazy-pages", Usage: "use userfaultfd to lazily restore memory pages"},
+		cli.StringFlag{Name: "status-fd", Value: "", Usage: "criu writes \\0 to this FD once lazy-pages is ready"},
 		cli.StringFlag{Name: "page-server", Value: "", Usage: "ADDRESS:PORT of the page server"},
 		cli.BoolFlag{Name: "file-locks", Usage: "handle file locks, for safety"},
 		cli.BoolFlag{Name: "pre-dump", Usage: "dump container's memory information only, leave the container running after this"},

libcontainer/container_linux.go

@@ -773,6 +773,25 @@ func (c *linuxContainer) addMaskPaths(req *criurpc.CriuReq) error {
 		}
 		req.Opts.ExtMnt = append(req.Opts.ExtMnt, extMnt)
 	}
+	return nil
+}
+
+func waitForCriuLazyServer(r *os.File, status string) error {
+
+	data := make([]byte, 1)
+	_, err := r.Read(data)
+	if err != nil {
+		return err
+	}
+	fd, err := os.OpenFile(status, os.O_TRUNC|os.O_WRONLY, os.ModeAppend)
+	if err != nil {
+		return err
+	}
+	_, err = fd.Write(data)
+	if err != nil {
+		return err
+	}
+	fd.Close()
 
 	return nil
 }
@@ -840,6 +859,7 @@ func (c *linuxContainer) Checkpoint(criuOpts *CriuOpts) error {
 		EmptyNs:         proto.Uint32(criuOpts.EmptyNs),
 		OrphanPtsMaster: proto.Bool(true),
 		AutoDedup:       proto.Bool(criuOpts.AutoDedup),
+		LazyPages:       proto.Bool(criuOpts.LazyPages),
 	}
 
 	fcg := c.cgroupManager.GetPaths()["freezer"]
@@ -890,6 +910,24 @@ func (c *linuxContainer) Checkpoint(criuOpts *CriuOpts) error {
 		Opts: &rpcOpts,
 	}
 
+	if criuOpts.LazyPages {
+		// lazy migration requested; check if criu supports it
+		feat := criurpc.CriuFeatures{
+			LazyPages: proto.Bool(true),
+		}
+
+		if err := c.checkCriuFeatures(criuOpts, &rpcOpts, &feat); err != nil {
+			return err
+		}
+
+		statusRead, statusWrite, err := os.Pipe()
+		if err != nil {
+			return err
+		}
+		rpcOpts.StatusFd = proto.Int32(int32(statusWrite.Fd()))
+		go waitForCriuLazyServer(statusRead, criuOpts.StatusFd)
+	}
+
 	//no need to dump these information in pre-dump
 	if !criuOpts.PreDump {
 		for _, m := range c.config.Mounts {
@@ -1042,6 +1080,7 @@ func (c *linuxContainer) Restore(process *Process, criuOpts *CriuOpts) error {
 			EmptyNs:         proto.Uint32(criuOpts.EmptyNs),
 			OrphanPtsMaster: proto.Bool(true),
 			AutoDedup:       proto.Bool(criuOpts.AutoDedup),
+			LazyPages:       proto.Bool(criuOpts.LazyPages),
 		},
 	}
 

libcontainer/criu_opts_linux.go

@@ -35,4 +35,6 @@ type CriuOpts struct {
 	ManageCgroupsMode       cgMode             // dump or restore cgroup mode
 	EmptyNs                 uint32             // don't c/r properties for namespace from this mask
 	AutoDedup               bool               // auto deduplication for incremental dumps
+	LazyPages               bool               // restore memory pages lazily using userfaultfd
+	StatusFd                string             // fd for feedback when lazy server is ready
 }

restore.go

@@ -86,6 +86,10 @@ using the runc checkpoint command.`,
 			Name:  "auto-dedup",
 			Usage: "enable auto deduplication of memory images",
 		},
+		cli.BoolFlag{
+			Name:  "lazy-pages",
+			Usage: "use userfaultfd to lazily restore memory pages",
+		},
 	},
 	Action: func(context *cli.Context) error {
 		if err := checkArgs(context, 1, exactArgs); err != nil {
@@ -128,5 +132,7 @@ func criuOptions(context *cli.Context) *libcontainer.CriuOpts {
 		FileLocks:               context.Bool("file-locks"),
 		PreDump:                 context.Bool("pre-dump"),
 		AutoDedup:               context.Bool("auto-dedup"),
+		LazyPages:               context.Bool("lazy-pages"),
+		StatusFd:                context.String("status-fd"),
 	}
 }