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+#!/usr/bin/env bash

+# Compute MAX_SWAPFILES by duplicating the computations found in swap.h as of 2023-08.

+

+function get_swap_define {

+	local value="${1}"

+	local kernel_release="$(uname -r | cut -d- -f1,2)"

+	local swap_h_path="/usr/src/linux-headers-${kernel_release}-common/include/linux/swap.h"

+	perl -nE 'say $1 if (m/#define\s+'${value}'\s+(\d+)/ && $1 > 0)' "${swap_h_path}"

+}

+

+function get_kernel_config {

+	local config="${1}"

+	local kernel_config="/boot/config-$(uname -r)"

+	grep -q "^${config}=y\$" "${kernel_config}"

+}

+

+MAX_SWAPFILES_SHIFT=$(get_swap_define MAX_SWAPFILES_SHIFT)

+SWP_DEVICE_NUM=$(get_swap_define SWP_DEVICE_NUM)

+SWP_MIGRATION_NUM=$(get_swap_define SWP_MIGRATION_NUM)

+SWP_HWPOISON_NUM=$(get_swap_define SWP_HWPOISON_NUM)

+SWP_PTE_MARKER_NUM=$(get_swap_define SWP_PTE_MARKER_NUM)

+

+((MAX_SWAPFILES=1 << MAX_SWAPFILES_SHIFT))

+kernel_config="/boot/config-$(uname -r)"

+get_kernel_config CONFIG_MIGRATION && ((MAX_SWAPFILES-=SWP_MIGRATION_NUM))

+get_kernel_config CONFIG_MEMORY_FAILURE && ((MAX_SWAPFILES-=SWP_HWPOISON_NUM))

+get_kernel_config CONFIG_DEVICE_PRIVATE && ((MAX_SWAPFILES-=SWP_DEVICE_NUM))

+((MAX_SWAPFILES-=SWP_PTE_MARKER_NUM))

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+#!/usr/bin/env bash

+# Compute MAX_SWAPFILES by "running" swap.h using the C preprocessor.

+

+kernel_config="/boot/config-$(uname -r)"

+kernel_release="$(uname -r | cut -d- -f1,2)"

+# Debian provides swap.h through package linux-headers-xxx-common:

+swap_h_path="/usr/src/linux-headers-${kernel_release}-common/include/linux/swap.h"

+MAX_SWAPFILES=$(

+	{

+		# Turn the current kernel's configuration into a sequence of #define statements:

+		perl -ne 'printf(qq[#define %s %s\n], $1, $2) if m#^([^=]+)=(.*)$#' "${kernel_config}"

+		# Append include/linux/swap.h but get rid of #include statements for the sake of simplicity:

+		grep -v '#include' "${swap_h_path}"

+		# Last line: instruct the C preprocessor to output the value of

+		# MAX_SWAPFILES; we expect a single-line math expression, written with C in

+		# mind yet Python/Perl-friendly, hence the addition of print() for Perl:

+		echo 'print(MAX_SWAPFILES)'

+	} | cpp | tail -1 | perl -

+	# ^ Feed the whole thing to the C preprocessor, keep only the last line and feed it to Perl

+)

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+#!/usr/bin/env bash

+# Create and enable as many 1-GiB swap files as the kernel allows.

+source determine-max-swapfiles-2.bash

+echo "MAX_SWAPFILES: ${MAX_SWAPFILES}"

+

+umask 0077

+hostname=$(hostname -s)

+((bs=1024**2))

+((size=1*1024*bs))

+((count=size/bs))

+for num in $(seq 1 "${MAX_SWAPFILES}"); do

+	printf -v num_padded "%02d" "${num}"

+	file="/data/swap/${num_padded}"

+	label="${hostname^^}SWAP${num_padded}"

+	# 4-KiB swap partition header:

+	truncate -s 4096 "${file}"

+	# Fill it with zeroes:

+	dd if=/dev/zero of="${file}" bs=4096 count=1 status=none

+	# Add the desired swap size, filled with zeroes too:

+	dd if=/dev/zero of="${file}" conv=notrunc oflag=append count="${count}" bs="${bs}" status=none

+	# Format the swap partition:

+	mkswap -L "${label}" "${file}"

+	swapon "${file}"

+done