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// 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/abi/linux"
"gvisor.dev/gvisor/pkg/sentry/fs/ext/disklayout"
"gvisor.dev/gvisor/pkg/syserror"
)
// 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
}
// newFilesystem is the filesystem constructor.
func newFilesystem(dev io.ReadSeeker) (*filesystem, error) {
fs := filesystem{dev: dev}
var err error
fs.sb, err = readSuperBlock(dev)
if err != nil {
return nil, err
}
if fs.sb.Magic() != linux.EXT_SUPER_MAGIC {
// mount(2) specifies that EINVAL should be returned if the superblock is
// invalid.
return nil, syserror.EINVAL
}
fs.bgs, err = readBlockGroups(dev, fs.sb)
if err != nil {
return nil, err
}
return &fs, nil
}
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