ext: Inode creation logic.

PiperOrigin-RevId: 259666476

1 files changed, 37 insertions, 11 deletions

diff --git a/pkg/sentry/fs/ext/ext.go b/pkg/sentry/fs/ext/ext.go
index 8bc591c8b..7f4287b01 100644
--- a/pkg/sentry/fs/ext/ext.go
+++ b/pkg/sentry/fs/ext/ext.go
@@ -26,21 +26,29 @@ import (
 
 // 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 serializes changes to the Dentry tree and the usage of the read seeker.
+	mu sync.Mutex
 
-	// 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.
+	// dev is the ReadSeeker for the underlying fs device. It is protected by mu.
+	//
+	// The ext filesystems aim to maximize locality, i.e. place all the data
+	// blocks of a file 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
+	dev io.ReadSeeker
+
+	// inodeCache maps absolute inode numbers to the corresponding Inode struct.
+	// Inodes should be removed from this once their reference count hits 0.
+	//
+	// Protected by mu because every addition and removal from this corresponds to
+	// a change in the dentry tree.
+	inodeCache map[uint32]*inode
 
 	// sb represents the filesystem superblock. Immutable after initialization.
 	sb disklayout.SuperBlock
@@ -52,7 +60,7 @@ type filesystem struct {
 
 // newFilesystem is the filesystem constructor.
 func newFilesystem(dev io.ReadSeeker) (*filesystem, error) {
-	fs := filesystem{dev: dev}
+	fs := filesystem{dev: dev, inodeCache: make(map[uint32]*inode)}
 	var err error
 
 	fs.sb, err = readSuperBlock(dev)
@@ -73,3 +81,21 @@ func newFilesystem(dev io.ReadSeeker) (*filesystem, error) {
 
 	return &fs, nil
 }
+
+// getOrCreateInode gets the inode corresponding to the inode number passed in.
+// It creates a new one with the given inode number if one does not exist.
+//
+// Preconditions: must be holding fs.mu.
+func (fs *filesystem) getOrCreateInode(inodeNum uint32) (*inode, error) {
+	if in, ok := fs.inodeCache[inodeNum]; ok {
+		return in, nil
+	}
+
+	in, err := newInode(fs.dev, fs.sb, fs.bgs, inodeNum)
+	if err != nil {
+		return nil, err
+	}
+
+	fs.inodeCache[inodeNum] = in
+	return in, nil
+}