Remove workspacefs loopback mount code (#8064)

We aren't actively using this code. The `debugfs` codepath is probably
"fast enough" since we updated it to only copy output paths in
#7959, and also this
code is likely to be supplanted by FUSE anyway. If we do want to try
this code again in the future, we can always bring it back, but should
test thoroughly in dev first because we hit some issues with the
loopback device approach last time.

enterprise/server/remote_execution/containers/firecracker/BUILD

@@ -49,7 +49,6 @@ go_library(
         "//server/interfaces",
         "//server/metrics",
         "//server/remote_cache/digest",
-        "//server/util/alert",
         "//server/util/background",
         "//server/util/disk",
         "//server/util/log",

enterprise/server/remote_execution/containers/firecracker/firecracker.go

@@ -48,7 +48,6 @@ import (
 	"github.com/buildbuddy-io/buildbuddy/server/interfaces"
 	"github.com/buildbuddy-io/buildbuddy/server/metrics"
 	"github.com/buildbuddy-io/buildbuddy/server/remote_cache/digest"
-	"github.com/buildbuddy-io/buildbuddy/server/util/alert"
 	"github.com/buildbuddy-io/buildbuddy/server/util/background"
 	"github.com/buildbuddy-io/buildbuddy/server/util/disk"
 	"github.com/buildbuddy-io/buildbuddy/server/util/log"
@@ -76,7 +75,6 @@ import (
 	hlpb "google.golang.org/grpc/health/grpc_health_v1"
 )
 
-var firecrackerMountWorkspaceFile = flag.Bool("executor.firecracker_mount_workspace_file", false, "Enables mounting workspace filesystem to improve performance of copying action outputs.")
 var firecrackerCgroupVersion = flag.String("executor.firecracker_cgroup_version", "", "Specifies the cgroup version for firecracker to use.")
 var debugStreamVMLogs = flag.Bool("executor.firecracker_debug_stream_vm_logs", false, "Stream firecracker VM logs to the terminal.")
 var debugTerminal = flag.Bool("executor.firecracker_debug_terminal", false, "Run an interactive terminal in the Firecracker VM connected to the executor's controlling terminal. For debugging only.")
@@ -551,16 +549,15 @@ type FirecrackerContainer struct {
 	uffdHandler *uffd.Handler
 	memoryStore *copy_on_write.COWStore
 
-	jailerRoot         string               // the root dir the jailer will work in
-	numaNode           int                  // NUMA node for CPU scheduling
-	cpuWeightMillis    int64                // milliCPU for cgroup CPU weight
-	cgroupParent       string               // parent cgroup path (root-relative)
-	cgroupSettings     *scpb.CgroupSettings // jailer cgroup settings
-	blockDevice        *block_io.Device     // block device for cgroup IO settings
-	machine            *fcclient.Machine    // the firecracker machine object.
-	vmLog              *VMLog
-	env                environment.Env
-	mountWorkspaceFile bool
+	jailerRoot      string               // the root dir the jailer will work in
+	numaNode        int                  // NUMA node for CPU scheduling
+	cpuWeightMillis int64                // milliCPU for cgroup CPU weight
+	cgroupParent    string               // parent cgroup path (root-relative)
+	cgroupSettings  *scpb.CgroupSettings // jailer cgroup settings
+	blockDevice     *block_io.Device     // block device for cgroup IO settings
+	machine         *fcclient.Machine    // the firecracker machine object.
+	vmLog           *VMLog
+	env             environment.Env
 
 	vmCtx context.Context
 	// cancelVmCtx cancels the Machine context, stopping the VMM if it hasn't
@@ -610,23 +607,22 @@ func NewContainer(ctx context.Context, env environment.Env, task *repb.Execution
 	}
 
 	c := &FirecrackerContainer{
-		vmConfig:           opts.VMConfiguration.CloneVT(),
-		executorConfig:     opts.ExecutorConfig,
-		jailerRoot:         opts.ExecutorConfig.JailerRoot,
-		containerImage:     opts.ContainerImage,
-		user:               opts.User,
-		actionWorkingDir:   opts.ActionWorkingDirectory,
-		cpuWeightMillis:    opts.CPUWeightMillis,
-		cgroupParent:       opts.CgroupParent,
-		cgroupSettings:     opts.CgroupSettings,
-		blockDevice:        opts.BlockDevice,
-		numaNode:           numaNode,
-		env:                env,
-		task:               task,
-		loader:             loader,
-		vmLog:              vmLog,
-		mountWorkspaceFile: *firecrackerMountWorkspaceFile,
-		cancelVmCtx:        func(err error) {},
+		vmConfig:         opts.VMConfiguration.CloneVT(),
+		executorConfig:   opts.ExecutorConfig,
+		jailerRoot:       opts.ExecutorConfig.JailerRoot,
+		containerImage:   opts.ContainerImage,
+		user:             opts.User,
+		actionWorkingDir: opts.ActionWorkingDirectory,
+		cpuWeightMillis:  opts.CPUWeightMillis,
+		cgroupParent:     opts.CgroupParent,
+		cgroupSettings:   opts.CgroupSettings,
+		blockDevice:      opts.BlockDevice,
+		numaNode:         numaNode,
+		env:              env,
+		task:             task,
+		loader:           loader,
+		vmLog:            vmLog,
+		cancelVmCtx:      func(err error) {},
 	}
 
 	c.vmConfig.KernelVersion = c.executorConfig.KernelVersion
@@ -1555,86 +1551,6 @@ func (c *FirecrackerContainer) vmLogWriter() io.Writer {
 	return c.vmLog
 }
 
-type loopMount struct {
-	loopControlFD *os.File
-	imageFD       *os.File
-	loopDevIdx    int
-	loopFD        *os.File
-	mountDir      string
-}
-
-func (m *loopMount) Unmount() {
-	if m.mountDir != "" {
-		if err := syscall.Unmount(m.mountDir, 0); err != nil {
-			alert.UnexpectedEvent("firecracker_could_not_unmount_loop_device", err.Error())
-		}
-		m.mountDir = ""
-	}
-	if m.loopDevIdx >= 0 && m.loopControlFD != nil {
-		err := unix.IoctlSetInt(int(m.loopControlFD.Fd()), unix.LOOP_CTL_REMOVE, m.loopDevIdx)
-		if err != nil {
-			alert.UnexpectedEvent("firecracker_could_not_release_loop_device", err.Error())
-		}
-		m.loopDevIdx = -1
-	}
-	if m.loopFD != nil {
-		m.loopFD.Close()
-		m.loopFD = nil
-	}
-	if m.imageFD != nil {
-		m.imageFD.Close()
-		m.imageFD = nil
-	}
-	if m.loopControlFD != nil {
-		m.loopControlFD.Close()
-		m.loopControlFD = nil
-	}
-}
-
-func mountExt4ImageUsingLoopDevice(imagePath string, mountTarget string) (lm *loopMount, retErr error) {
-	loopControlFD, err := os.Open("/dev/loop-control")
-	if err != nil {
-		return nil, err
-	}
-	defer loopControlFD.Close()
-
-	m := &loopMount{loopDevIdx: -1}
-	defer func() {
-		if retErr != nil {
-			m.Unmount()
-		}
-	}()
-
-	imageFD, err := os.Open(imagePath)
-	if err != nil {
-		return nil, err
-	}
-	m.imageFD = imageFD
-
-	loopDevIdx, err := unix.IoctlRetInt(int(loopControlFD.Fd()), unix.LOOP_CTL_GET_FREE)
-	if err != nil {
-		return nil, status.UnknownErrorf("could not allocate loop device: %s", err)
-	}
-	m.loopDevIdx = loopDevIdx
-
-	loopDevicePath := fmt.Sprintf("/dev/loop%d", loopDevIdx)
-	loopFD, err := os.OpenFile(loopDevicePath, os.O_RDWR, 0)
-	if err != nil {
-		return nil, err
-	}
-	m.loopFD = loopFD
-
-	if err := unix.IoctlSetInt(int(loopFD.Fd()), unix.LOOP_SET_FD, int(imageFD.Fd())); err != nil {
-		return nil, status.UnknownErrorf("could not set loop device FD: %s", err)
-	}
-
-	if err := syscall.Mount(loopDevicePath, mountTarget, "ext4", unix.MS_RDONLY, "norecovery"); err != nil {
-		return nil, err
-	}
-	m.mountDir = mountTarget
-	return m, nil
-}
-
 // copyOutputsToWorkspace copies output files from the workspace filesystem
 // image to the local filesystem workdir. It does not overwrite existing files.
 //
@@ -1671,18 +1587,9 @@ func (c *FirecrackerContainer) copyOutputsToWorkspace(ctx context.Context) error
 	}
 	defer os.RemoveAll(wsDir) // clean up
 
-	if c.mountWorkspaceFile {
-		m, err := mountExt4ImageUsingLoopDevice(workspaceExt4Path, wsDir)
-		if err != nil {
-			log.CtxWarningf(ctx, "could not mount ext4 image: %s", err)
-			return err
-		}
-		defer m.Unmount()
-	} else {
-		outputPaths := workspacePathsToExtract(c.task)
-		if err := ext4.ImageToDirectory(ctx, workspaceExt4Path, wsDir, outputPaths); err != nil {
-			return err
-		}
+	outputPaths := workspacePathsToExtract(c.task)
+	if err := ext4.ImageToDirectory(ctx, workspaceExt4Path, wsDir, outputPaths); err != nil {
+		return err
 	}
 
 	walkErr := fs.WalkDir(os.DirFS(wsDir), ".", func(path string, d fs.DirEntry, err error) error {