armbian-kernel.sh: introduce KERNEL_BTF=no to opt-out of BTF/CO-RE type-info on low-RAM machines

- turns out `pahole` for `vmlinux` can take multiple gigabytes of RAM to run successfully
- I can't simply decide based on available RAM, as that would make .config hashes mismatch
- thus, introduce:
  - default is to enable BTF; if on low-ram host, error out unless KERNEL_BTF=yes is passed
  - if KERNEL_BTF=no is passed, the BTF debug info is always disabled
  - if KERNEL_BTF=yes is passed, then a warning is produced, but BTF is still enabled
- the magic number "6451 MiB" was determined empirically (and is probably bs)

Author: rpardini

lib/functions/compilation/armbian-kernel.sh

@@ -57,51 +57,46 @@ function armbian_kernel_config__disable_various_options() {
 	fi
 }
 
-function armbian_kernel_config__600_enable_ebpf_and_btf_info() {
-	display_alert "Enabling eBPF and BTF info" "for fully BTF & CO-RE enabled kernel" "info"
-
+function armbian_kernel_config__force_pa_va_48_bits_on_arm64() {
 	declare -A opts_val=()
-	declare -a opts_n=("CONFIG_DEBUG_INFO_NONE")
-	declare -a opts_y=(
-		"CONFIG_BPF_JIT" "CONFIG_BPF_JIT_DEFAULT_ON" "CONFIG_FTRACE_SYSCALLS" "CONFIG_PROBE_EVENTS_BTF_ARGS" "CONFIG_BPF_KPROBE_OVERRIDE"
-		"CONFIG_DEBUG_INFO" "CONFIG_DEBUG_INFO_DWARF5"
-		"CONFIG_DEBUG_INFO_BTF" "CONFIG_DEBUG_INFO_BTF_MODULES"
-	)
-
+	declare -a opts_y=() opts_n=()
 	if [[ "${ARCH}" == "arm64" ]]; then
 		opts_y+=("CONFIG_ARM64_VA_BITS_48")
 		opts_val["CONFIG_ARM64_PA_BITS"]="48"
 	fi
+	armbian_kernel_config_apply_opts_from_arrays
+}
 
-	declare opt_y opt_val opt_n
-	for opt_n in "${opts_n[@]}"; do
-		kernel_config_modifying_hashes+=("${opt_n}=n")
-	done
-
-	for opt_y in "${opts_y[@]}"; do
-		kernel_config_modifying_hashes+=("${opt_y}=y")
-	done
-
-	for opt_val in "${!opts_val[@]}"; do
-		kernel_config_modifying_hashes+=("${opt_val}=${opts_val[$opt_val]}")
-	done
-
-	if [[ -f .config ]]; then
-		for opt_n in "${opts_n[@]}"; do
-			display_alert "Disabling kernel opt" "${opt_n}=n" "debug"
-			kernel_config_set_n "${opt_n}"
-		done
+function armbian_kernel_config__600_enable_ebpf_and_btf_info() {
+	declare -A opts_val=()
+	declare -a opts_y=() opts_n=()
 
-		for opt_y in "${opts_y[@]}"; do
-			display_alert "Enabling kernel opt" "${opt_y}=y" "debug"
-			kernel_config_set_y "${opt_y}"
-		done
+	if [[ "${KERNEL_BTF}" == "no" ]]; then # If user is explicit by passing "KERNEL_BTF=no", then actually disable all debug info.
+		display_alert "Disabling eBPF and BTF info for kernel" "as requested by KERNEL_BTF=no" "info"
+		opts_y+=("CONFIG_DEBUG_INFO_NONE")                                                                               # Enable the "none" option
+		opts_n+=("CONFIG_DEBUG_INFO" "CONFIG_DEBUG_INFO_DWARF5" "CONFIG_DEBUG_INFO_BTF" "CONFIG_DEBUG_INFO_BTF_MODULES") # BTF & CO-RE == off
+		# We don't disable the eBPF options, as eBPF itself doesn't require BTF (debug info) and doesnt' consume as much memory during build as BTF debug info does.
+	else
+		declare -i available_physical_memory_mib
+		available_physical_memory_mib=$(($(awk '/MemAvailable/ {print $2}' /proc/meminfo) / 1024)) # MiB
+		display_alert "Considering available RAM for BTF build" "${available_physical_memory_mib} MiB" "info"
+
+		if [[ ${available_physical_memory_mib} -lt 6451 ]]; then # If less than 6451 MiB of RAM is available, then exit with an error, telling the user to avoid pain and set KERNEL_BTF=no ...
+			if [[ "${KERNEL_BTF}" == "yes" ]]; then                 # ... except if the user knows better, and has set KERNEL_BTF=yes, then we'll just warn.
+				display_alert "Not enough RAM available (${available_physical_memory_mib}Mib) for BTF build" "but KERNEL_BTF=yes is set; enabling BTF" "warn"
+			else
+				exit_with_error "Not enough RAM available (${available_physical_memory_mib}Mib) for BTF build. Please set 'KERNEL_BTF=no' to avoid running out of memory during the kernel LD/BTF build step; or ignore this check by setting 'KERNEL_BTF=yes' -- that might put a lot of load on your swap disk, if any."
+			fi
+		fi
 
-		for opt_val in "${!opts_val[@]}"; do
-			display_alert "Setting kernel opt" "${opt_val}=${opts_val[$opt_val]}" "debug"
-			kernel_config_set_val "${opt_val}" "${opts_val[$opt_val]}"
-		done
+		display_alert "Enabling eBPF and BTF info" "for fully BTF & CO-RE enabled kernel" "info"
+		opts_n+=("CONFIG_DEBUG_INFO_NONE") # Make sure the "none" option is disabled
+		opts_y+=(
+			"CONFIG_BPF_JIT" "CONFIG_BPF_JIT_DEFAULT_ON" "CONFIG_FTRACE_SYSCALLS" "CONFIG_PROBE_EVENTS_BTF_ARGS" "CONFIG_BPF_KPROBE_OVERRIDE" # eBPF == on
+			"CONFIG_DEBUG_INFO" "CONFIG_DEBUG_INFO_DWARF5" "CONFIG_DEBUG_INFO_BTF" "CONFIG_DEBUG_INFO_BTF_MODULES"                            # BTF & CO-RE == off
+		)
 	fi
+	armbian_kernel_config_apply_opts_from_arrays
 
 	return 0
 }
@@ -166,3 +161,37 @@ function kernel_config_set_val() {
 	display_alert "Setting kernel config/module value" "${config}=${value}" "debug"
 	run_host_command_logged ./scripts/config --set-val "${config}" "${value}"
 }
+
+# This takes opts_n, opts_y, arrays from parent scope; also the opts_val dictionary;
+# it and applies them to the hashes and to the .config if it exists.
+function armbian_kernel_config_apply_opts_from_arrays() {
+	declare opt_y opt_val opt_n
+	for opt_n in "${opts_n[@]}"; do
+		kernel_config_modifying_hashes+=("${opt_n}=n")
+	done
+
+	for opt_y in "${opts_y[@]}"; do
+		kernel_config_modifying_hashes+=("${opt_y}=y")
+	done
+
+	for opt_val in "${!opts_val[@]}"; do
+		kernel_config_modifying_hashes+=("${opt_val}=${opts_val[$opt_val]}")
+	done
+
+	if [[ -f .config ]]; then
+		for opt_n in "${opts_n[@]}"; do
+			display_alert "Disabling kernel opt" "${opt_n}=n" "debug"
+			kernel_config_set_n "${opt_n}"
+		done
+
+		for opt_y in "${opts_y[@]}"; do
+			display_alert "Enabling kernel opt" "${opt_y}=y" "debug"
+			kernel_config_set_y "${opt_y}"
+		done
+
+		for opt_val in "${!opts_val[@]}"; do
+			display_alert "Setting kernel opt" "${opt_val}=${opts_val[$opt_val]}" "debug"
+			kernel_config_set_val "${opt_val}" "${opts_val[$opt_val]}"
+		done
+	fi
+}