WSL2-Linux-Kernel/Documentation/power/swsusp.txt

Some warnings, first.

 * BIG FAT WARNING *********************************************************
 *
 * If you touch anything on disk between suspend and resume...
 *				...kiss your data goodbye.
 *
 * If you do resume from initrd after your filesystems are mounted...
 *				...bye bye root partition.
 *			[this is actually same case as above]
 *
 * If you have unsupported (*) devices using DMA, you may have some
 * problems. If your disk driver does not support suspend... (IDE does),
 * it may cause some problems, too. If you change kernel command line
 * between suspend and resume, it may do something wrong. If you change
 * your hardware while system is suspended... well, it was not good idea;
 * but it will probably only crash.
 *
 * (*) suspend/resume support is needed to make it safe.
 *
 * If you have any filesystems on USB devices mounted before software suspend,
 * they won't be accessible after resume and you may lose data, as though
 * you have unplugged the USB devices with mounted filesystems on them;
 * see the FAQ below for details.  (This is not true for more traditional
 * power states like "standby", which normally don't turn USB off.)

Swap partition:
You need to append resume=/dev/your_swap_partition to kernel command
line or specify it using /sys/power/resume.

Swap file:
If using a swapfile you can also specify a resume offset using
resume_offset=<number> on the kernel command line or specify it
in /sys/power/resume_offset.

After preparing then you suspend by

echo shutdown > /sys/power/disk; echo disk > /sys/power/state

. If you feel ACPI works pretty well on your system, you might try

echo platform > /sys/power/disk; echo disk > /sys/power/state

. If you would like to write hibernation image to swap and then suspend
to RAM (provided your platform supports it), you can try

echo suspend > /sys/power/disk; echo disk > /sys/power/state

. If you have SATA disks, you'll need recent kernels with SATA suspend
support. For suspend and resume to work, make sure your disk drivers
are built into kernel -- not modules. [There's way to make
suspend/resume with modular disk drivers, see FAQ, but you probably
should not do that.]

If you want to limit the suspend image size to N bytes, do

echo N > /sys/power/image_size

before suspend (it is limited to around 2/5 of available RAM by default).

. The resume process checks for the presence of the resume device,
if found, it then checks the contents for the hibernation image signature.
If both are found, it resumes the hibernation image.

. The resume process may be triggered in two ways:
  1) During lateinit:  If resume=/dev/your_swap_partition is specified on
     the kernel command line, lateinit runs the resume process.  If the
     resume device has not been probed yet, the resume process fails and
     bootup continues.
  2) Manually from an initrd or initramfs:  May be run from
     the init script by using the /sys/power/resume file.  It is vital
     that this be done prior to remounting any filesystems (even as
     read-only) otherwise data may be corrupted.

Article about goals and implementation of Software Suspend for Linux
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Author: Gábor Kuti
Last revised: 2003-10-20 by Pavel Machek

Idea and goals to achieve

Nowadays it is common in several laptops that they have a suspend button. It
saves the state of the machine to a filesystem or to a partition and switches
to standby mode. Later resuming the machine the saved state is loaded back to
ram and the machine can continue its work. It has two real benefits. First we
save ourselves the time machine goes down and later boots up, energy costs
are real high when running from batteries. The other gain is that we don't have to
interrupt our programs so processes that are calculating something for a long
time shouldn't need to be written interruptible.

swsusp saves the state of the machine into active swaps and then reboots or
powerdowns.  You must explicitly specify the swap partition to resume from with
``resume='' kernel option. If signature is found it loads and restores saved
state. If the option ``noresume'' is specified as a boot parameter, it skips
the resuming.  If the option ``hibernate=nocompress'' is specified as a boot
parameter, it saves hibernation image without compression.

In the meantime while the system is suspended you should not add/remove any
of the hardware, write to the filesystems, etc.

Sleep states summary
====================

There are three different interfaces you can use, /proc/acpi should
work like this:

In a really perfect world:
echo 1 > /proc/acpi/sleep       # for standby
echo 2 > /proc/acpi/sleep       # for suspend to ram
echo 3 > /proc/acpi/sleep       # for suspend to ram, but with more power conservative
echo 4 > /proc/acpi/sleep       # for suspend to disk
echo 5 > /proc/acpi/sleep       # for shutdown unfriendly the system

and perhaps
echo 4b > /proc/acpi/sleep      # for suspend to disk via s4bios

Frequently Asked Questions
==========================

Q: well, suspending a server is IMHO a really stupid thing,
but... (Diego Zuccato):

A: You bought new UPS for your server. How do you install it without
bringing machine down? Suspend to disk, rearrange power cables,
resume.

You have your server on UPS. Power died, and UPS is indicating 30
seconds to failure. What do you do? Suspend to disk.


Q: Maybe I'm missing something, but why don't the regular I/O paths work?

A: We do use the regular I/O paths. However we cannot restore the data
to its original location as we load it. That would create an
inconsistent kernel state which would certainly result in an oops.
Instead, we load the image into unused memory and then atomically copy
it back to it original location. This implies, of course, a maximum
image size of half the amount of memory.

There are two solutions to this:

* require half of memory to be free during suspend. That way you can
read "new" data onto free spots, then cli and copy

* assume we had special "polling" ide driver that only uses memory
between 0-640KB. That way, I'd have to make sure that 0-640KB is free
during suspending, but otherwise it would work...

suspend2 shares this fundamental limitation, but does not include user
data and disk caches into "used memory" by saving them in
advance. That means that the limitation goes away in practice.

Q: Does linux support ACPI S4?

A: Yes. That's what echo platform > /sys/power/disk does.

Q: What is 'suspend2'?

A: suspend2 is 'Software Suspend 2', a forked implementation of
suspend-to-disk which is available as separate patches for 2.4 and 2.6
kernels from swsusp.sourceforge.net. It includes support for SMP, 4GB
highmem and preemption. It also has a extensible architecture that
allows for arbitrary transformations on the image (compression,
encryption) and arbitrary backends for writing the image (eg to swap
or an NFS share[Work In Progress]). Questions regarding suspend2
should be sent to the mailing list available through the suspend2
website, and not to the Linux Kernel Mailing List. We are working
toward merging suspend2 into the mainline kernel.

Q: What is the freezing of tasks and why are we using it?

A: The freezing of tasks is a mechanism by which user space processes and some
kernel threads are controlled during hibernation or system-wide suspend (on some
architectures).  See freezing-of-tasks.txt for details.

Q: What is the difference between "platform" and "shutdown"?

A:

shutdown: save state in linux, then tell bios to powerdown

platform: save state in linux, then tell bios to powerdown and blink
          "suspended led"

"platform" is actually right thing to do where supported, but
"shutdown" is most reliable (except on ACPI systems).

Q: I do not understand why you have such strong objections to idea of
selective suspend.

A: Do selective suspend during runtime power management, that's okay. But
it's useless for suspend-to-disk. (And I do not see how you could use
it for suspend-to-ram, I hope you do not want that).

Lets see, so you suggest to

* SUSPEND all but swap device and parents
* Snapshot
* Write image to disk
* SUSPEND swap device and parents
* Powerdown

Oh no, that does not work, if swap device or its parents uses DMA,
you've corrupted data. You'd have to do

* SUSPEND all but swap device and parents
* FREEZE swap device and parents
* Snapshot
* UNFREEZE swap device and parents
* Write
* SUSPEND swap device and parents

Which means that you still need that FREEZE state, and you get more
complicated code. (And I have not yet introduce details like system
devices).

Q: There don't seem to be any generally useful behavioral
distinctions between SUSPEND and FREEZE.

A: Doing SUSPEND when you are asked to do FREEZE is always correct,
but it may be unnecessarily slow. If you want your driver to stay simple,
slowness may not matter to you. It can always be fixed later.

For devices like disk it does matter, you do not want to spindown for
FREEZE.

Q: After resuming, system is paging heavily, leading to very bad interactivity.

A: Try running

cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u | while read file
do
  test -f "$file" && cat "$file" > /dev/null
done

after resume. swapoff -a; swapon -a may also be useful.

Q: What happens to devices during swsusp? They seem to be resumed
during system suspend?

A: That's correct. We need to resume them if we want to write image to
disk. Whole sequence goes like

      Suspend part
      ~~~~~~~~~~~~
      running system, user asks for suspend-to-disk

      user processes are stopped

      suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
      		      with state snapshot

      state snapshot: copy of whole used memory is taken with interrupts disabled

      resume(): devices are woken up so that we can write image to swap

      write image to swap

      suspend(PMSG_SUSPEND): suspend devices so that we can power off

      turn the power off

      Resume part
      ~~~~~~~~~~~
      (is actually pretty similar)

      running system, user asks for suspend-to-disk

      user processes are stopped (in common case there are none, but with resume-from-initrd, no one knows)

      read image from disk

      suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
      		      with image restoration

      image restoration: rewrite memory with image

      resume(): devices are woken up so that system can continue

      thaw all user processes

Q: What is this 'Encrypt suspend image' for?

A: First of all: it is not a replacement for dm-crypt encrypted swap.
It cannot protect your computer while it is suspended. Instead it does
protect from leaking sensitive data after resume from suspend.

Think of the following: you suspend while an application is running
that keeps sensitive data in memory. The application itself prevents
the data from being swapped out. Suspend, however, must write these
data to swap to be able to resume later on. Without suspend encryption
your sensitive data are then stored in plaintext on disk.  This means
that after resume your sensitive data are accessible to all
applications having direct access to the swap device which was used
for suspend. If you don't need swap after resume these data can remain
on disk virtually forever. Thus it can happen that your system gets
broken in weeks later and sensitive data which you thought were
encrypted and protected are retrieved and stolen from the swap device.
To prevent this situation you should use 'Encrypt suspend image'.

During suspend a temporary key is created and this key is used to
encrypt the data written to disk. When, during resume, the data was
read back into memory the temporary key is destroyed which simply
means that all data written to disk during suspend are then
inaccessible so they can't be stolen later on.  The only thing that
you must then take care of is that you call 'mkswap' for the swap
partition used for suspend as early as possible during regular
boot. This asserts that any temporary key from an oopsed suspend or
from a failed or aborted resume is erased from the swap device.

As a rule of thumb use encrypted swap to protect your data while your
system is shut down or suspended. Additionally use the encrypted
suspend image to prevent sensitive data from being stolen after
resume.

Q: Can I suspend to a swap file?

A: Generally, yes, you can.  However, it requires you to use the "resume=" and
"resume_offset=" kernel command line parameters, so the resume from a swap file
cannot be initiated from an initrd or initramfs image.  See
swsusp-and-swap-files.txt for details.

Q: Is there a maximum system RAM size that is supported by swsusp?

A: It should work okay with highmem.

Q: Does swsusp (to disk) use only one swap partition or can it use
multiple swap partitions (aggregate them into one logical space)?

A: Only one swap partition, sorry.

Q: If my application(s) causes lots of memory & swap space to be used
(over half of the total system RAM), is it correct that it is likely
to be useless to try to suspend to disk while that app is running?

A: No, it should work okay, as long as your app does not mlock()
it. Just prepare big enough swap partition.

Q: What information is useful for debugging suspend-to-disk problems?

A: Well, last messages on the screen are always useful. If something
is broken, it is usually some kernel driver, therefore trying with as
little as possible modules loaded helps a lot. I also prefer people to
suspend from console, preferably without X running. Booting with
init=/bin/bash, then swapon and starting suspend sequence manually
usually does the trick. Then it is good idea to try with latest
vanilla kernel.

Q: How can distributions ship a swsusp-supporting kernel with modular
disk drivers (especially SATA)?

A: Well, it can be done, load the drivers, then do echo into
/sys/power/resume file from initrd. Be sure not to mount
anything, not even read-only mount, or you are going to lose your
data.

Q: How do I make suspend more verbose?

A: If you want to see any non-error kernel messages on the virtual
terminal the kernel switches to during suspend, you have to set the
kernel console loglevel to at least 4 (KERN_WARNING), for example by
doing

	# save the old loglevel
	read LOGLEVEL DUMMY < /proc/sys/kernel/printk
	# set the loglevel so we see the progress bar.
	# if the level is higher than needed, we leave it alone.
	if [ $LOGLEVEL -lt 5 ]; then
	        echo 5 > /proc/sys/kernel/printk
		fi

        IMG_SZ=0
        read IMG_SZ < /sys/power/image_size
        echo -n disk > /sys/power/state
        RET=$?
        #
        # the logic here is:
        # if image_size > 0 (without kernel support, IMG_SZ will be zero),
        # then try again with image_size set to zero.
	if [ $RET -ne 0 -a $IMG_SZ -ne 0 ]; then # try again with minimal image size
                echo 0 > /sys/power/image_size
                echo -n disk > /sys/power/state
                RET=$?
        fi

	# restore previous loglevel
	echo $LOGLEVEL > /proc/sys/kernel/printk
	exit $RET

Q: Is this true that if I have a mounted filesystem on a USB device and
I suspend to disk, I can lose data unless the filesystem has been mounted
with "sync"?

A: That's right ... if you disconnect that device, you may lose data.
In fact, even with "-o sync" you can lose data if your programs have
information in buffers they haven't written out to a disk you disconnect,
or if you disconnect before the device finished saving data you wrote.

Software suspend normally powers down USB controllers, which is equivalent
to disconnecting all USB devices attached to your system.

Your system might well support low-power modes for its USB controllers
while the system is asleep, maintaining the connection, using true sleep
modes like "suspend-to-RAM" or "standby".  (Don't write "disk" to the
/sys/power/state file; write "standby" or "mem".)  We've not seen any
hardware that can use these modes through software suspend, although in
theory some systems might support "platform" modes that won't break the
USB connections.

Remember that it's always a bad idea to unplug a disk drive containing a
mounted filesystem.  That's true even when your system is asleep!  The
safest thing is to unmount all filesystems on removable media (such USB,
Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays)
before suspending; then remount them after resuming.

There is a work-around for this problem.  For more information, see
Documentation/driver-api/usb/persist.rst.

Q: Can I suspend-to-disk using a swap partition under LVM?

A: Yes and No.  You can suspend successfully, but the kernel will not be able
to resume on its own.  You need an initramfs that can recognize the resume
situation, activate the logical volume containing the swap volume (but not
touch any filesystems!), and eventually call

echo -n "$major:$minor" > /sys/power/resume

where $major and $minor are the respective major and minor device numbers of
the swap volume.

uswsusp works with LVM, too.  See http://suspend.sourceforge.net/

Q: I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were
compiled with the similar configuration files. Anyway I found that
suspend to disk (and resume) is much slower on 2.6.16 compared to
2.6.15. Any idea for why that might happen or how can I speed it up?

A: This is because the size of the suspend image is now greater than
for 2.6.15 (by saving more data we can get more responsive system
after resume).

There's the /sys/power/image_size knob that controls the size of the
image.  If you set it to 0 (eg. by echo 0 > /sys/power/image_size as
root), the 2.6.15 behavior should be restored.  If it is still too
slow, take a look at suspend.sf.net -- userland suspend is faster and
supports LZF compression to speed it up further.