swsusp.txt

linux 内核源代码

cat `cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u` > /dev/null

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, noone 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/disk/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/usb/persist.txt.

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.