mount: auto-enable Linux FUSE passthrough for read-only files

On a read-only open, when the kernel supports FUSE_PASSTHROUGH (Linux >= 6.9, privileged mount), materialize the file into a local backing file and register it via RegisterBackingFd so the kernel serves reads and mmap directly from the backing file, bypassing the mount process.

No opt-in flag: support is probed by the first RegisterBackingFd call and latched off on failure, so unsupported mounts transparently use the normal read path. Gated to read-only opens of files up to -fusePassthroughMaxMB (default 256; 0 disables). The backing is shared per inode and torn down when the last open is released; MaxStackDepth is set so the kernel accepts registration.

Tradeoff: a passthrough handle reads a point-in-time snapshot, so concurrent writes are not reflected while the file stays open - best for immutable / read-mostly data. The data path is unchanged when disabled or unsupported, and the feature is a no-op on non-Linux platforms.

weed/command/mount_std.go

@@ -315,6 +315,11 @@ func RunMount(option *MountOptions, umask os.FileMode) bool {
 	if option.cacheSymlink != nil && *option.cacheSymlink {
 		fuseMountOptions.EnableSymlinkCaching = true
 	}
+	if option.fusePassthroughMaxMB != nil && *option.fusePassthroughMaxMB > 0 {
+		// FUSE_PASSTHROUGH requires a non-zero stacking depth advertised in
+		// FUSE_INIT; without it the kernel rejects backing-file registration.
+		fuseMountOptions.MaxStackDepth = 1
+	}
 
 	// find mount point
 	mountRoot := filerMountRootPath
@@ -328,6 +333,11 @@ func RunMount(option *MountOptions, umask os.FileMode) bool {
 		cacheDirForWrite = cacheDirForRead
 	}
 
+	passthroughMaxMB := int64(0)
+	if option.fusePassthroughMaxMB != nil {
+		passthroughMaxMB = *option.fusePassthroughMaxMB
+	}
+
 	seaweedFileSystem := mount.NewSeaweedFileSystem(&mount.Option{
 		MountDirectory:              dir,
 		FilerAddresses:              filerAddresses,
@@ -373,6 +383,7 @@ func RunMount(option *MountOptions, umask os.FileMode) bool {
 		EnableDistributedLock: option.distributedLock != nil && *option.distributedLock,
 		WritebackCache:        option.writebackCache != nil && *option.writebackCache,
 		PosixDirNlink:         option.posixDirNlink != nil && *option.posixDirNlink,
+		FusePassthroughMaxMB:  passthroughMaxMB,
 		// Peer chunk sharing
 		PeerEnabled:    option.peerEnabled != nil && *option.peerEnabled,
 		PeerListen:     peerStringOrEmpty(option.peerListen),