Allow dcache=0 in runsc to completely disable dentry caching in gofer.

PiperOrigin-RevId: 472621428

pkg/sentry/fsimpl/gofer/gofer.go

@@ -113,13 +113,18 @@ type dentryCache struct {
 
 // SetDentryCacheSize sets the size of the global gofer dentry cache.
 func SetDentryCacheSize(size int) {
-	globalDentryCache.mu.Lock()
-	defer globalDentryCache.mu.Unlock()
-	globalDentryCache.maxCachedDentries = uint64(size)
+	if size < 0 {
+		return
+	}
+	if globalDentryCache != nil {
+		log.Warningf("Global dentry cache has already been initialized. Ignoring subsequent attempt.")
+		return
+	}
+	globalDentryCache = &dentryCache{maxCachedDentries: uint64(size)}
 }
 
 // globalDentryCache is a global cache of dentries across all gofers.
-var globalDentryCache dentryCache
+var globalDentryCache *dentryCache
 
 // Valid values for "trans" mount option.
 const transportModeFD = "fd"
@@ -494,13 +499,11 @@ func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 	}
 
 	// Did the user configure a global dentry cache?
-	globalDentryCache.mu.Lock()
-	if globalDentryCache.maxCachedDentries >= 1 {
-		fs.dentryCache = &globalDentryCache
+	if globalDentryCache != nil {
+		fs.dentryCache = globalDentryCache
 	} else {
 		fs.dentryCache = &dentryCache{maxCachedDentries: defaultMaxCachedDentries}
 	}
-	globalDentryCache.mu.Unlock()
 
 	fs.vfsfs.Init(vfsObj, &fstype, fs)
 

runsc/config/flags.go

@@ -84,7 +84,7 @@ func RegisterFlags(flagSet *flag.FlagSet) {
 	flagSet.Bool("cgroupfs", false, "Automatically mount cgroupfs.")
 	flagSet.Bool("ignore-cgroups", false, "don't configure cgroups.")
 	flagSet.Int("fdlimit", -1, "Specifies a limit on the number of host file descriptors that can be open. Applies separately to the sentry and gofer. Note: each file in the sandbox holds more than one host FD open.")
-	flagSet.Int("dcache", 0, "Set the global gofer direct cache size. This acts as a coarse-grained control on the number of host FDs simultaneously open by the sentry. If zero, per-mount caches are used.")
+	flagSet.Int("dcache", -1, "Set the global dentry cache size. This acts as a coarse-grained control on the number of host FDs simultaneously open by the sentry. If negative, per-mount caches are used.")
 
 	// Flags that control sandbox runtime behavior: network related.
 	flagSet.Var(networkTypePtr(NetworkSandbox), "network", "specifies which network to use: sandbox (default), host, none. Using network inside the sandbox is more secure because it's isolated from the host network.")