ext: boilerplate code.

Renamed ext4 to ext since we are targeting ext(2/3/4).
Removed fs.go since we are targeting VFS2.
Added ext.go with filesystem struct.

PiperOrigin-RevId: 257689775

1 files changed, 49 insertions, 0 deletions

diff --git a/pkg/sentry/fs/ext/ext.go b/pkg/sentry/fs/ext/ext.go
new file mode 100644
index 000000000..7602e2bf0
--- /dev/null
+++ b/pkg/sentry/fs/ext/ext.go
@@ -0,0 +1,49 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package ext implements readonly ext(2/3/4) filesystems.
+package ext
+
+import (
+	"io"
+	"sync"
+
+	"gvisor.dev/gvisor/pkg/sentry/fs/ext/disklayout"
+)
+
+// Filesystem implements vfs.FilesystemImpl.
+type Filesystem struct {
+	// dev is the ReadSeeker for the underlying fs device and is protected by mu.
+	dev io.ReadSeeker
+
+	// mu synchronizes the usage of dev. The ext filesystems take locality into
+	// condsideration, i.e. data blocks of a file will tend to be placed close
+	// together. On a spinning disk, locality reduces the amount of movement of
+	// the head hence speeding up IO operations. On an SSD there are no moving
+	// parts but locality increases the size of each transer request. Hence,
+	// having mutual exclusion on the read seeker while reading a file *should*
+	// help in achieving the intended performance gains.
+	//
+	// Note: This synchronization was not coupled with the ReadSeeker itself
+	// because we want to synchronize across read/seek operations for the
+	// performance gains mentioned above. Helps enforcing one-file-at-a-time IO.
+	mu sync.Mutex
+
+	// sb represents the filesystem superblock. Immutable after initialization.
+	sb disklayout.SuperBlock
+
+	// bgs represents all the block group descriptors for the filesystem.
+	// Immutable after initialization.
+	bgs []disklayout.BlockGroup
+}