overlay[2]: rm fs checks

Now that we do check if overlay is working by performing an actual
overlayfs mount, there's no need in extra checks for the kernel version
or the filesystem type. Actual mount check is sufficient.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

daemon/graphdriver/overlay/overlay.go

@@ -136,20 +136,6 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 		return nil, graphdriver.ErrNotSupported
 	}
 
-	fsMagic, err := graphdriver.GetFSMagic(testdir)
-	if err != nil {
-		return nil, err
-	}
-	if fsName, ok := graphdriver.FsNames[fsMagic]; ok {
-		backingFs = fsName
-	}
-
-	switch fsMagic {
-	case graphdriver.FsMagicAufs, graphdriver.FsMagicBtrfs, graphdriver.FsMagicEcryptfs, graphdriver.FsMagicNfsFs, graphdriver.FsMagicOverlay, graphdriver.FsMagicZfs:
-		logrus.WithField("storage-driver", "overlay").Errorf("'overlay' is not supported over %s", backingFs)
-		return nil, graphdriver.ErrIncompatibleFS
-	}
-
 	supportsDType, err := fsutils.SupportsDType(testdir)
 	if err != nil {
 		return nil, err

daemon/graphdriver/overlay2/overlay.go

@@ -27,7 +27,6 @@ import (
 	"github.com/docker/docker/pkg/locker"
 	"github.com/docker/docker/pkg/mount"
 	"github.com/docker/docker/pkg/parsers"
-	"github.com/docker/docker/pkg/parsers/kernel"
 	"github.com/docker/docker/pkg/system"
 	units "github.com/docker/go-units"
 	rsystem "github.com/opencontainers/runc/libcontainer/system"
@@ -132,12 +131,6 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 		return nil, err
 	}
 
-	// require kernel 4.0.0 to ensure multiple lower dirs are supported
-	v, err := kernel.GetKernelVersion()
-	if err != nil {
-		return nil, err
-	}
-
 	// Perform feature detection on /var/lib/docker/overlay2 if it's an existing directory.
 	// This covers situations where /var/lib/docker/overlay2 is a mount, and on a different
 	// filesystem than /var/lib/docker.
@@ -152,30 +145,6 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 		return nil, graphdriver.ErrNotSupported
 	}
 
-	fsMagic, err := graphdriver.GetFSMagic(testdir)
-	if err != nil {
-		return nil, err
-	}
-	if fsName, ok := graphdriver.FsNames[fsMagic]; ok {
-		backingFs = fsName
-	}
-
-	switch fsMagic {
-	case graphdriver.FsMagicAufs, graphdriver.FsMagicEcryptfs, graphdriver.FsMagicNfsFs, graphdriver.FsMagicOverlay, graphdriver.FsMagicZfs:
-		logger.Errorf("'overlay2' is not supported over %s", backingFs)
-		return nil, graphdriver.ErrIncompatibleFS
-	case graphdriver.FsMagicBtrfs:
-		// Support for OverlayFS on BTRFS was added in kernel 4.7
-		// See https://btrfs.wiki.kernel.org/index.php/Changelog
-		if kernel.CompareKernelVersion(*v, kernel.VersionInfo{Kernel: 4, Major: 7, Minor: 0}) < 0 {
-			if !opts.overrideKernelCheck {
-				logger.Errorf("'overlay2' requires kernel 4.7 to use on %s", backingFs)
-				return nil, graphdriver.ErrIncompatibleFS
-			}
-			logger.Warn("Using pre-4.7.0 kernel for overlay2 on btrfs, may require kernel update")
-		}
-	}
-
 	supportsDType, err := fsutils.SupportsDType(testdir)
 	if err != nil {
 		return nil, err