bug#54783: [PATCH 1/2] system: Align zram priority with swap-space spec to clarify.

[Josselin Poiret]

* gnu/services/linux.scm
(zram-device-configuration) [priority]: Adapt to use #f or an integer
from 0 to 32767.  Add sanitizer to warn for the change and delay the
field.
(zram-device-configuration->udev-string): Adapt as above.
* doc/guix.texi (Zram Device Service): Change priority description to
refer to the Swap Space one, and suggest not leaving the default #f on
to properly use zram.
---
 doc/guix.texi          | 10 +++++-----
 gnu/services/linux.scm | 26 +++++++++++++++++++++++---
 2 files changed, 28 insertions(+), 8 deletions(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index b7005f0ef1..31f391357d 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -96,7 +96,7 @@ Copyright @copyright{} 2021 Domagoj Stolfa@*
 Copyright @copyright{} 2021 Hui Lu@*
 Copyright @copyright{} 2021 pukkamustard@*
 Copyright @copyright{} 2021 Alice Brenon@*
-Copyright @copyright{} 2021 Josselin Poiret@*
+Copyright @copyright{} 2021, 2022 Josselin Poiret@*
 Copyright @copyright{} 2021 Andrew Tropin@*
 Copyright @copyright{} 2021 Sarah Morgensen@*
 Copyright @copyright{} 2021 Josselin Poiret@*
@@ -34650,11 +34650,11 @@ 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.: @code{"2G"}.
-@item @code{priority} (default @code{-1})
+@item @code{priority} (default @code{#f})
 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.
+@xref{Swap Space} for a description of swap priorities.  You might want
+to set a specific priority for the zram device, otherwise it could end
+up not being used much for the reasons described there.
 @end table
 
 @end deftp
diff --git a/gnu/services/linux.scm b/gnu/services/linux.scm
index 2eb02ac5a3..9f598b2826 100644
--- a/gnu/services/linux.scm
+++ b/gnu/services/linux.scm
@@ -4,6 +4,7 @@
 ;;; Copyright © 2020 Efraim Flashner <efraim@flashner.co.il>
 ;;; Copyright © 2021 raid5atemyhomework <raid5atemyhomework@protonmail.com>
 ;;; Copyright © 2021 B. Wilson <elaexuotee@wilsonb.com>
+;;; Copyright © 2022 Josselin Poiret <dev@jpoiret.xyz>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -21,9 +22,11 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (gnu services linux)
+  #:use-module (guix diagnostics)
   #:use-module (guix gexp)
   #:use-module (guix records)
   #:use-module (guix modules)
+  #:use-module (guix i18n)
   #:use-module (gnu services)
   #:use-module (gnu services base)
   #:use-module (gnu services shepherd)
@@ -252,7 +255,19 @@ (define-record-type* <zram-device-configuration>
   (memory-limit             zram-device-configuration-memory-limit
                             (default 0))        ; string or integer
   (priority                 zram-device-configuration-priority
-                            (default -1)))      ; integer
+                            (default #f)        ; integer | #f
+                            (delayed)
+                            (sanitize warn-zram-priority-change)))
+
+(define-with-syntax-properties
+  (warn-zram-priority-change (priority properties))
+  (if (eqv? priority -1)
+      (begin
+        (warning (source-properties->location properties)
+                 (G_ "Using -1 for zram priority is deprecated to align with \
+the corresponding swap-space field, please use #f from now on.~%"))
+        #f)
+      priority))
 
 (define (zram-device-configuration->udev-string config)
   "Translate a <zram-device-configuration> into a string which can be
@@ -278,8 +293,13 @@ (define (zram-device-configuration->udev-string config)
          "")
        "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) " ")
+       ;; XXX: The field is delayed while the deprecation warning remains in
+       ;; place, so we can't use match to fetch it (it would give a promise)
+       (if (zram-device-configuration-priority config)
+           (string-append "--priority "
+                          (number->string
+                           (zram-device-configuration-priority config))
+                          " ")
          "")
        "/dev/zram0\"\n"))))
 
-- 
2.34.0