linux-boot: Mount /dev as a devtmpfs from the start.

Suggested by Petter <petter@mykolab.ch>
and Mark H Weaver <mhw@netris.org>.
Reported by Duncan Keall <duncan@duncankeall.com>.

Partly fixes <http://bugs.gnu.org/19190> by populating /dev/mapper
early enough.

* gnu/build/linux-boot.scm (mount-essential-file-systems): Mount /dev as
  a devtmpfs.
  (move-essential-file-systems): Add /dev.
  (mount-root-file-system): Mount /rw-root/dev as a devtmpfs instead of
  calling 'make-essential-device-nodes'.
  (boot-system): Remove call to 'make-essential-device-nodes'.
* gnu/system/file-systems.scm (%devtmpfs-file-system): Remove.
* doc/guix.texi (File Systems): Adjust accordingly.

doc/guix.texi

@@ -5295,16 +5295,11 @@ variables.
 
 @defvr {Scheme Variable} %base-file-systems
 These are essential file systems that are required on normal systems,
-such as @var{%devtmpfs-file-system} and @var{%immutable-store} (see
+such as @var{%pseudo-terminal-file-system} and @var{%immutable-store} (see
 below.)  Operating system declarations should always contain at least
 these.
 @end defvr
 
-@defvr {Scheme Variable} %devtmpfs-file-system
-The @code{devtmpfs} file system to be mounted on @file{/dev}.  This is a
-requirement for udev (@pxref{Base Services, @code{udev-service}}).
-@end defvr
-
 @defvr {Scheme Variable} %pseudo-terminal-file-system
 This is the file system to be mounted as @file{/dev/pts}.  It supports
 @dfn{pseudo-terminals} created @i{via} @code{openpty} and similar

gnu/build/linux-boot.scm

@@ -48,7 +48,7 @@
 ;;; Code:
 
 (define* (mount-essential-file-systems #:key (root "/"))
-  "Mount /proc and /sys under ROOT."
+  "Mount /dev, /proc, and /sys under ROOT."
   (define (scope dir)
     (string-append root
                    (if (string-suffix? "/" root)
@@ -60,6 +60,10 @@
     (mkdir (scope "proc")))
   (mount "none" (scope "proc") "proc")
 
+  (unless (file-exists? (scope "dev"))
+    (mkdir (scope "dev")))
+  (mount "none" (scope "dev") "devtmpfs")
+
   (unless (file-exists? (scope "sys"))
     (mkdir (scope "sys")))
   (mount "none" (scope "sys") "sysfs"))
@@ -71,7 +75,7 @@
                 (unless (file-exists? target)
                   (mkdir target))
                 (mount dir target "" MS_MOVE)))
-            '("/proc" "/sys")))
+            '("/dev" "/proc" "/sys")))
 
 (define (linux-command-line)
   "Return the Linux kernel command line as a list of strings."
@@ -100,7 +104,7 @@ with the given MAJOR number, starting with MINOR."
 
 (define* (make-essential-device-nodes #:key (root "/"))
   "Make essential device nodes under ROOT/dev."
-  ;; The hand-made udev!
+  ;; The hand-made devtmpfs/udev!
 
   (define (scope dir)
     (string-append root
@@ -255,7 +259,8 @@ UNIONFS."
         (mount "none" "/rw-root" "tmpfs")
 
         ;; We want read-write /dev nodes.
-        (make-essential-device-nodes #:root "/rw-root")
+        (mkdir-p "/rw-root/dev")
+        (mount "none" "/rw-root/dev" "devtmpfs")
 
         ;; Make /root a union of the tmpfs and the actual root.  Use
         ;; 'max_files' to set a high RLIMIT_NOFILE for the unionfs process
@@ -385,9 +390,6 @@ to it are lost."
          (unless (configure-qemu-networking)
            (display "network interface is DOWN\n")))
 
-       ;; Make /dev nodes.
-       (make-essential-device-nodes)
-
        ;; Prepare the real root file system under /root.
        (unless (file-exists? "/root")
          (mkdir "/root"))
@@ -405,10 +407,6 @@ to it are lost."
                                    #:volatile-root? volatile-root?)
            (mount "none" "/root" "tmpfs"))
 
-       (unless (file-exists? "/root/dev")
-         (mkdir "/root/dev")
-         (make-essential-device-nodes #:root "/root"))
-
        ;; Mount the specified file systems.
        (for-each mount-file-system
                  (remove root-mount-point? mounts))

gnu/system/file-systems.scm

@@ -47,7 +47,6 @@
             %binary-format-file-system
             %shared-memory-file-system
             %pseudo-terminal-file-system
-            %devtmpfs-file-system
             %immutable-store
             %control-groups
             %elogind-file-systems
@@ -186,17 +185,6 @@ UUID representation."
     (type "binfmt_misc")
     (check? #f)))
 
-(define %devtmpfs-file-system
-  ;; /dev as a 'devtmpfs' file system, needed for udev.
-  (file-system
-    (device "none")
-    (mount-point "/dev")
-    (type "devtmpfs")
-    (check? #f)
-
-    ;; Mount it from the initrd so /dev/pts & co. can then be mounted over it.
-    (needed-for-boot? #t)))
-
 (define %tty-gid
   ;; ID of the 'tty' group.  Allocate it statically to make it easy to refer
   ;; to it from here and from the 'tty' group definitions.
@@ -282,8 +270,7 @@ UUID representation."
 (define %base-file-systems
   ;; List of basic file systems to be mounted.  Note that /proc and /sys are
   ;; currently mounted by the initrd.
-  (append (list %devtmpfs-file-system
-                %pseudo-terminal-file-system
+  (append (list %pseudo-terminal-file-system
                 %shared-memory-file-system
                 %immutable-store)
           %elogind-file-systems