[systemd-devel] [RFC] [PATCHv2 0/3] resume: implement support for resuming from hibernation

[Ivan Shapovalov]

This patchset allows systemd to parse resume= kernel command line parameter
and initiate resume from the specified device.

It adds:
- a 'systemd-resume' tool which takes path to a device node and
  writes its major:minor to /sys/power/state;
- a corresponding 'systemd-resume at .service' templated unit;
- a 'systemd-resume-generator' generator which parses the kernel command line
  and instantiates the unit as necessary.

This functionality already exists in-kernel, but only for "/dev/sdXY"-style
pathes. Implementing it in userspace allows to use arbitrary udev-created
symlinks, e. g. persistent block device pathes ("/dev/disk/by-foo/bar").

Userspace parsing of resume= kernel command line parameter has been
traditionally done in initramfs via shell scripts (for Arch Linux, this is
"resume" mkinitcpio hook), so I feel that this feature has its place within
systemd.

Due to the nature of hibernation, the resume unit must be activated before
any modifications to filesystems take place. This can happen only in initramfs
before mounting anything.

So, first patch orders all non-root fsck after local-fs-pre.target, which in
turn allows to order the resume unit before those fsck instances.

Second and third patches add the tool, the unit and the generator.

There are some issues with this implementation.

- legacy usr.mount is not automatically ordered after local-fs-pre.target,
  so systemd-resume at .service has to be manually ordered before it;
- systemd-udevd.service, which is needed for creating persistent block device
  symlinks, is transitively ordered after systemd-remount-fs.service via at
  least systemd-udev-hwdb-update.service and systemd-sysusers.service.
  Hence, if these units are present, an ordering cycle happens and resume is
  impossible.

Encountering these conditions is very unlikely inside of an initramfs (who
would make the in-initramfs /usr external? who would add systemd-sysusers
into initramfs?), but there can still be pathological cases.

So, I would like someone to comment on these issues.

This is my first patch to this project, so feel free to flak me for missing
something obvious :)

Thanks for reviewing!

v2: fix issues pointed out by Andrei:
    - don't RemainAfterExit because it's useless
    - don't attempt to resume outside of initramfs because it's unsafe
      (reiserfs replays journal even if mounted RO)

Ivan Shapovalov (3):
  units: order systemd-fsck at .service after local-fs-pre.target.
  resume: add a tool to write a device node's major:minor to /sys/power/resume.
  resume-generator: add a generator for instantiating the resume unit.

 Makefile-man.am                         |  9 ++++
 Makefile.am                             | 28 ++++++++--
 man/kernel-command-line.xml             | 13 ++++-
 man/systemd-resume-generator.xml        | 91 ++++++++++++++++++++++++++++++++
 man/systemd-resume at .service.xml         | 81 ++++++++++++++++++++++++++++
 src/resume-generator/Makefile           |  1 +
 src/resume-generator/resume-generator.c | 93 +++++++++++++++++++++++++++++++++
 src/resume/Makefile                     |  1 +
 src/resume/resume.c                     | 82 +++++++++++++++++++++++++++++
 units/systemd-fsck at .service.in          |  2 +-
 units/systemd-resume at .service.in        | 20 +++++++
 11 files changed, 415 insertions(+), 6 deletions(-)
 create mode 100644 man/systemd-resume-generator.xml
 create mode 100644 man/systemd-resume at .service.xml
 create mode 120000 src/resume-generator/Makefile
 create mode 100644 src/resume-generator/resume-generator.c
 create mode 120000 src/resume/Makefile
 create mode 100644 src/resume/resume.c
 create mode 100644 units/systemd-resume at .service.in

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2.1.0