implement symlink caching

connection.go

@@ -148,6 +148,8 @@ func (c *Connection) Init() error {
 		c.protocol = initOp.Kernel
 	}
 
+	cacheSymlinks := initOp.Flags&fusekernel.InitCacheSymlinks > 0
+
 	// Respond to the init op.
 	initOp.Library = c.protocol
 	initOp.MaxReadahead = maxReadahead
@@ -163,6 +165,12 @@ func (c *Connection) Init() error {
 		initOp.Flags |= fusekernel.InitWritebackCache
 	}
 
+	// Enable caching symlink targets in the kernel page cache if the user opted
+	// into it (might require fixing the size field of inode attributes first):
+	if c.cfg.EnableSymlinkCaching && cacheSymlinks {
+		initOp.Flags |= fusekernel.InitCacheSymlinks
+	}
+
 	c.Reply(ctx, nil)
 	return nil
 }

internal/fusekernel/fuse_kernel.go

@@ -266,6 +266,7 @@ const (
 	InitAsyncDIO        InitFlags = 1 << 15
 	InitWritebackCache  InitFlags = 1 << 16
 	InitNoOpenSupport   InitFlags = 1 << 17
+	InitCacheSymlinks   InitFlags = 1 << 23
 
 	InitCaseSensitive InitFlags = 1 << 29 // OS X only
 	InitVolRename     InitFlags = 1 << 30 // OS X only
@@ -296,6 +297,7 @@ var initFlagNames = []flagName{
 	{uint32(InitAsyncDIO), "InitAsyncDIO"},
 	{uint32(InitWritebackCache), "InitWritebackCache"},
 	{uint32(InitNoOpenSupport), "InitNoOpenSupport"},
+	{uint32(InitCacheSymlinks), "InitCacheSymlinks"},
 
 	{uint32(InitCaseSensitive), "InitCaseSensitive"},
 	{uint32(InitVolRename), "InitVolRename"},

mount_config.go

@@ -128,6 +128,17 @@ type MountConfig struct {
 	// entries will be cached for an arbitrarily long time.
 	EnableVnodeCaching bool
 
+	// Linux only.
+	//
+	// Linux 4.20 introduced caching symlink targets in the page cache:
+	// https://github.com/torvalds/linux/commit/5571f1e65486be025f73fa6aa30fb03725d362a2
+	//
+	// This is not enabled by default because the old behavior masked a bug:
+	// file systems could return any size in the inode attributes of
+	// symlinks. After enabling caching, the specified size caps the symlink
+	// target.
+	EnableSymlinkCaching bool
+
 	// OS X only.
 	//
 	// The name of the mounted volume, as displayed in the Finder. If empty, a