services: Add zram-device-service.

* gnu/services/linux.scm (<zram-device-configuration>): New record.
(zram-device-service-type): New variable.
* doc/guix.texi (Linux Services): Document it.
* tests/services/linux.scm (zram-swap-device-test): New tests.

doc/guix.texi

@@ -27283,6 +27283,51 @@ parameters, can be done as follow:
 @end lisp
 @end deffn
 
+@cindex zram
+@cindex compressed swap
+@cindex Compressed RAM-based block devices
+@subsubheading Zram Device Service
+
+The Zram device service provides a compressed swap device in system
+memory.  The Linux Kernel documentation has more information about
+@uref{https://www.kernel.org/doc/html/latest/admin-guide/blockdev/zram.html,zram}
+devices.
+
+@deffn {Scheme Variable} zram-device-service-type
+This service creates the zram block device, formats it as swap and
+enables it as a swap device.  The service's value is a
+@code{zram-device-configuration} record.
+
+@deftp {Data Type} zram-device-configuration
+This is the data type representing the configuration for the zram-device
+service.
+
+@table @asis
+@item @code{size} (default @var{"1G"})
+This is the amount of space you wish to provide for the zram device.  It
+accepts a string and can be a number of bytes or use a suffix, eg.:
+@var{"512M"} or @var{1024000}.
+@item @code{compression-algorithm} (default @var{'lzo})
+This is the compression algorithm you wish to use.  It is difficult to
+list all the possible compression options, but common ones supported by
+Guix's Linux Libre Kernel include @var{'lzo}, @var{'lz4} and @var{'zstd}.
+@item @code{memory-limit} (default @var{0})
+This is the maximum amount of memory which the zram device can use.
+Setting it to '0' disables the limit.  While it is generally expected
+that compression will be 2:1, it is possible that uncompressable data
+can be written to swap and this is a method to limit how much memory can
+be used.  It accepts a string and can be a number of bytes or use a
+suffix, eg.: @var{"2G"}.
+@item @code{priority} (default @var{-1})
+This is the priority of the swap device created from the zram device.
+@code{swapon} accepts values between -1 and 32767, with higher values
+indicating higher priority.  Higher priority swap will generally be used
+first.
+@end table
+
+@end deftp
+@end deffn
+
 @node Hurd Services
 @subsection Hurd Services
 

gnu/services/linux.scm

@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;; Copyright © 2020 Brice Waegeneire <brice@waegenei.re>
+;;; Copyright © 2020 Efraim Flashner <efraim@flashner.co.il>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -22,6 +23,7 @@ (define-module (gnu services linux)
   #:use-module (guix records)
   #:use-module (guix modules)
   #:use-module (gnu services)
+  #:use-module (gnu services base)
   #:use-module (gnu services shepherd)
   #:use-module (gnu packages linux)
   #:use-module (srfi srfi-1)
@@ -42,7 +44,15 @@ (define-module (gnu services linux)
             earlyoom-configuration-send-notification-command
             earlyoom-service-type
 
-            kernel-module-loader-service-type))
+            kernel-module-loader-service-type
+
+            zram-device-configuration
+            zram-device-configuration?
+            zram-device-configuration-size
+            zram-device-configuration-compression-algorithm
+            zram-device-configuration-memory-limit
+            zram-device-configuration-priority
+            zram-device-service-type))
 
 
 ;;;
@@ -177,3 +187,72 @@ (define kernel-module-loader-service-type
    (compose concatenate)
    (extend append)
    (default-value '())))
+
+
+;;;
+;;; Kernel module loader.
+;;;
+
+(define-record-type* <zram-device-configuration>
+  zram-device-configuration make-zram-device-configuration
+  zram-device-configuration?
+  (size                     zram-device-configration-size
+                            (default "1G"))     ; string or integer
+  (compression-algorithm    zram-device-configuration-compression-algorithm
+                            (default 'lzo))     ; symbol
+  (memory-limit             zram-device-configuration-memory-limit
+                            (default 0))        ; string or integer
+  (priority                 zram-device-configuration-priority
+                            (default -1)))      ; integer
+
+(define (zram-device-configuration->udev-string config)
+  "Translate a <zram-device-configuration> into a string which can be
+placed in a udev rules file."
+  (match config
+    (($ <zram-device-configuration> size compression-algorithm memory-limit priority)
+     (string-append
+       "KERNEL==\"zram0\", "
+       "ATTR{comp_algorithm}=\"" (symbol->string compression-algorithm) "\" "
+       (if (not (or (equal? "0" size)
+                    (equal? 0 size)))
+         (string-append "ATTR{disksize}=\"" (if (number? size)
+                                              (number->string size)
+                                              size)
+                        "\" ")
+         "")
+       (if (not (or (equal? "0" memory-limit)
+                    (equal? 0 memory-limit)))
+         (string-append "ATTR{mem_limit}=\"" (if (number? memory-limit)
+                                               (number->string memory-limit)
+                                               memory-limit)
+                        "\" ")
+         "")
+       "RUN+=\"/run/current-system/profile/sbin/mkswap /dev/zram0\" "
+       "RUN+=\"/run/current-system/profile/sbin/swapon "
+       (if (not (equal? -1 priority))
+         (string-append "--priority " (number->string priority) " ")
+         "")
+       "/dev/zram0\"\n"))))
+
+(define %zram-device-config
+  `("modprobe.d/zram.conf"
+    ,(plain-file "zram.conf"
+                 "options zram num_devices=1")))
+
+(define (zram-device-udev-rule config)
+  (file->udev-rule "99-zram.rules"
+                   (plain-file "99-zram.rules"
+                               (zram-device-configuration->udev-string config))))
+
+(define zram-device-service-type
+  (service-type
+    (name 'zram)
+    (default-value (zram-device-configuration))
+    (extensions
+      (list (service-extension kernel-module-loader-service-type
+                               (const (list "zram")))
+            (service-extension etc-service-type
+                               (const (list %zram-device-config)))
+            (service-extension udev-service-type
+                               (compose list zram-device-udev-rule))))
+    (description "Creates a zram swap device.")))

tests/services/linux.scm

@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
+;;; Copyright © 2020 Efraim Flashner <efraim@flashner.co.il>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -54,4 +55,40 @@ (define %earlyoom-configuration-sample
         "-N" "python \"/some/path/notify-all-users.py\"")
   (earlyoom-configuration->command-line-args %earlyoom-configuration-sample))
 
+
+;;;
+;;; Zram swap device.
+;;;
+
+(define zram-device-configuration->udev-string
+  (@@ (gnu services linux) zram-device-configuration->udev-string))
+
+(define %zram-swap-device-test-1
+  (zram-device-configuration
+    (size "2G")
+    (compression-algorithm 'zstd)
+    (memory-limit "1G")
+    (priority 42)))
+
+(test-equal "zram-swap-device-test-1"
+  "KERNEL==\"zram0\", ATTR{comp_algorithm}=\"zstd\" ATTR{disksize}=\"2G\" ATTR{mem_limit}=\"1G\" RUN+=\"/run/current-system/profile/sbin/mkswap /dev/zram0\" RUN+=\"/run/current-system/profile/sbin/swapon --priority 42 /dev/zram0\"\n"
+  (zram-device-configuration->udev-string %zram-swap-device-test-1))
+
+(define %zram-swap-device-test-2
+  (zram-device-configuration
+    (size 1048576)  ; 1M
+    (compression-algorithm 'lz4)))
+
+(test-equal "zram-swap-device-test-2"
+  "KERNEL==\"zram0\", ATTR{comp_algorithm}=\"lz4\" ATTR{disksize}=\"1048576\" RUN+=\"/run/current-system/profile/sbin/mkswap /dev/zram0\" RUN+=\"/run/current-system/profile/sbin/swapon /dev/zram0\"\n"
+  (zram-device-configuration->udev-string %zram-swap-device-test-2))
+
+(define %zram-swap-device-test-3
+  (zram-device-configuration
+    (memory-limit (* 512 1000))))
+
+(test-equal "zram-swap-device-test-3"
+  "KERNEL==\"zram0\", ATTR{comp_algorithm}=\"lzo\" ATTR{disksize}=\"1G\" ATTR{mem_limit}=\"512000\" RUN+=\"/run/current-system/profile/sbin/mkswap /dev/zram0\" RUN+=\"/run/current-system/profile/sbin/swapon /dev/zram0\"\n"
+  (zram-device-configuration->udev-string %zram-swap-device-test-3))
+
 (test-end "linux-services")