hi,

arm64 is currently broken without swap, processes are "randomly"
getting killed.  this is very easy to reproduce by booting without
swap.  daemons such as smtpd or ntpd, or the kernel/lib relink will
crash.

this is caused by one of the following switch cases in
do_el0_sync() (arm64/trap.c), which is called by "handle_el0_sync":

370         switch (exception) {

397         case EXCP_INSN_ABORT_L:
398                 udata_abort(frame, esr, far, 1);
399                 break;

410         case EXCP_DATA_ABORT_L:
411                 udata_abort(frame, esr, far, 0);
412                 break;

these call udata_abort(), which, among other things, calls uvm_fault():

110         /* Handle referenced/modified emulation */
111         if (pmap_fault_fixup(map->pmap, va, access_type))
112                 return;
113
114         error = uvm_fault(map, va, 0, access_type);
115
116         if (error == 0) {
117                 uvm_grow(p, va);
118                 return;
119         }
120
121         if (error == ENOMEM) {
122                 sig = SIGKILL;
123                 code = 0;

134         sv.sival_ptr = (void *)far;
135         trapsignal(p, sig, esr, code, sv);

uvm_fault() fails with ENOMEM and udata_aobrt() then sends SIGKILL to
the process.

in uvm_fault(), there are only two functions which could return
ENOMEM: uvm_fault_lower() and uvm_fault_upper().  both of these
functions have branches, that, if they *think* that they are out
of memory, check if uvm_swapisfull().  if that is the case, they
return with ENOMEM, otherwise they wait for more RAM.  of course,
uvm_swapisfull() always returns true if there is no swap!

i have observed a few interesting thing when building the kernel
on arm64 without swap.  first, make without any "-j" flags works,
no processes are getting killed.  second, when running with make
"-j2" or "-j4", processes are going to get killed, usually cc.
however, after repeating this for three or four times, the build
will work without processes getting killed.

i have tested this issue on an "Raspberry Pi 4 Model B" and
Firefly iCore-3588Q ("MNT RK3588 Processor Module").

i want to debug this further, since i have to investigate another,
probably related, pmap/uvm issue anyways.  however, before that i
wanted to ask if someone has dug into this issue or has any clues
on what could be going on.  thanks!