Time Machine on Linux with Samba (4.11). Install avahi and this should 'just work'.

smb.conf

[global]
	workgroup = Time Machine
	netbios name = Time Machine
	security = user
	passdb backend = tdbsam

	# Supposedly turning signing off may increase performance for TM.
	# May require changes on client side, set to auto if issues occur.
	# See: https://support.apple.com/en-us/HT205926
	# Note that this is disabling a security feature. Only use on a
	# "trusted" connection (like your home network).
	server signing = no

	# Many guides also suggest modifying the buf options, but Samba's 
	# official documentation advises against this as manual tuning
	# is often more effort than its worth compared to modern kernel
	# auto tuning. Also consider IPTOS_THROUGHPUT.
	# Note that IPTOS_* options are Layer 3 QoS options, so if you're
	# on the same LAN, these will likely make no difference.
	socket options = IPTOS_LOWDELAY

	# This is just to have a neat little icon in Finder.
	vfs objects = catia fruit streams_xattr
	fruit:model = TimeCapsule

[Tempore Ex Machina]
	comment = Time Machine
	path = /mnt/backup
	browseable = yes
	writeable = yes
	create mask = 0600
	directory mask = 0700

	vfs objects = catia fruit streams_xattr
	fruit:aapl = yes
	fruit:time machine = yes
	fruit:metadata = stream
	fruit:veto_appledouble = no
	fruit:wipe_intentionally_left_blank_rfork = yes
	fruit:delete_empty_adfiles = yes
	# No idea if this actually works, there's apparently significant setup
	# needed outside of smb, but nothing breaks horribly so far with it on.
	spotlight = yes

	# Pass larger chunks directly to kernel recvfile, as well as use async I/O.
	# Unsure if these options counter each other, but these seemed to have a
	# significant improvement.
	min receivefile size = 16384
	aio write size = 16384
	aio read size = 16384