Remove filesystem structure from vfs.Dentry.

This change:

- Drastically simplifies the synchronization model: filesystem structure is
  both implementation-defined and implementation-synchronized.

- Allows implementations of vfs.DentryImpl to use implementation-specific
  dentry types, reducing casts during path traversal.

- Doesn't require dentries representing non-directory files to waste space on a
  map of children.

- Allows dentry revalidation and mount lookup to be correctly ordered (fixed
  FIXME in fsimpl/gofer/filesystem.go).

- Removes the need to have two separate maps in gofer.dentry
  (dentry.vfsd.children and dentry.negativeChildren) for positive and negative
  lookups respectively.

//pkg/sentry/fsimpl/tmpfs/benchmark_test.go:
name                        old time/op  new time/op  delta
VFS2TmpfsStat/1-112          172ns ± 4%   165ns ± 3%   -4.08%  (p=0.002 n=9+9)
VFS2TmpfsStat/2-112          199ns ± 3%   195ns ±10%     ~     (p=0.132 n=8+9)
VFS2TmpfsStat/3-112          230ns ± 2%   216ns ± 2%   -6.15%  (p=0.000 n=8+8)
VFS2TmpfsStat/8-112          390ns ± 2%   358ns ± 4%   -8.33%  (p=0.000 n=9+8)
VFS2TmpfsStat/64-112        2.20µs ± 3%  2.01µs ± 3%   -8.48%  (p=0.000 n=10+8)
VFS2TmpfsStat/100-112       3.42µs ± 9%  3.08µs ± 2%   -9.82%  (p=0.000 n=9+8)
VFS2TmpfsMountStat/1-112     278ns ± 1%   286ns ±15%     ~     (p=0.712 n=8+10)
VFS2TmpfsMountStat/2-112     311ns ± 4%   298ns ± 2%   -4.27%  (p=0.000 n=9+8)
VFS2TmpfsMountStat/3-112     339ns ± 3%   330ns ± 9%     ~     (p=0.070 n=8+9)
VFS2TmpfsMountStat/8-112     503ns ± 3%   466ns ± 3%   -7.38%  (p=0.000 n=8+8)
VFS2TmpfsMountStat/64-112   2.53µs ±16%  2.17µs ± 7%  -14.19%  (p=0.000 n=10+9)
VFS2TmpfsMountStat/100-112  3.60µs ± 4%  3.30µs ± 8%   -8.33%  (p=0.001 n=8+9)

Updates #1035

PiperOrigin-RevId: 307655892

1 files changed, 11 insertions, 10 deletions

diff --git a/pkg/sentry/fsimpl/ext/directory.go b/pkg/sentry/fsimpl/ext/directory.go
index bd6ede995..12b875c8f 100644
--- a/pkg/sentry/fsimpl/ext/directory.go
+++ b/pkg/sentry/fsimpl/ext/directory.go
@@ -21,7 +21,6 @@ import (
 	"gvisor.dev/gvisor/pkg/log"
 	"gvisor.dev/gvisor/pkg/sentry/fs"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/ext/disklayout"
-	"gvisor.dev/gvisor/pkg/sentry/memmap"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 	"gvisor.dev/gvisor/pkg/sync"
 	"gvisor.dev/gvisor/pkg/syserror"
@@ -31,6 +30,10 @@ import (
 type directory struct {
 	inode inode
 
+	// childCache maps filenames to dentries for children for which dentries
+	// have been instantiated. childCache is protected by filesystem.mu.
+	childCache map[string]*dentry
+
 	// mu serializes the changes to childList.
 	// Lock Order (outermost locks must be taken first):
 	//   directory.mu
@@ -50,9 +53,13 @@ type directory struct {
 	childMap map[string]*dirent
 }
 
-// newDirectroy is the directory constructor.
-func newDirectroy(inode inode, newDirent bool) (*directory, error) {
-	file := &directory{inode: inode, childMap: make(map[string]*dirent)}
+// newDirectory is the directory constructor.
+func newDirectory(inode inode, newDirent bool) (*directory, error) {
+	file := &directory{
+		inode:      inode,
+		childCache: make(map[string]*dentry),
+		childMap:   make(map[string]*dirent),
+	}
 	file.inode.impl = file
 
 	// Initialize childList by reading dirents from the underlying file.
@@ -299,9 +306,3 @@ func (fd *directoryFD) Seek(ctx context.Context, offset int64, whence int32) (in
 	fd.off = offset
 	return offset, nil
 }
-
-// ConfigureMMap implements vfs.FileDescriptionImpl.ConfigureMMap.
-func (fd *directoryFD) ConfigureMMap(ctx context.Context, opts *memmap.MMapOpts) error {
-	// mmap(2) specifies that EACCESS should be returned for non-regular file fds.
-	return syserror.EACCES
-}