-From kernel/suspend.c:
+Some warnings, first.
* BIG FAT WARNING *********************************************************
*
- * If you have unsupported (*) devices using DMA...
- * ...say goodbye to your data.
- *
* If you touch anything on disk between suspend and resume...
* ...kiss your data goodbye.
*
- * If your disk driver does not support suspend... (IDE does)
- * ...you'd better find out how to get along
- * without your data.
+ * If you do resume from initrd after your filesystems are mounted...
+ * ...bye bye root partition.
+ * [this is actually same case as above]
*
- * If you change kernel command line between suspend and resume...
- * ...prepare for nasty fsck or worse.
+ * 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.)
You need to append resume=/dev/your_swap_partition to kernel command
line. Then you suspend by
echo platform > /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 500 MB by default).
Article about goals and implementation of Software Suspend for Linux
You have your server on UPS. Power died, and UPS is indicating 30
seconds to failure. What do you do? Suspend to disk.
-Ethernet card in your server died. You want to replace it. Your
-server is not hotplug capable. What do you do? Suspend to disk,
-replace ethernet card, resume. If you are fast your users will not
-even see broken connections.
-
Q: Maybe I'm missing something, but why don't the regular I/O paths work?
A: Yes. That's what echo platform > /sys/power/disk does.
-Q: My machine doesn't work with ACPI. How can I use swsusp than ?
-
-A: Do a reboot() syscall with right parameters. Warning: glibc gets in
-its way, so check with strace:
-
-reboot(LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, 0xd000fce2)
-
-(Thanks to Peter Osterlund:)
-
-#include <unistd.h>
-#include <syscall.h>
-
-#define LINUX_REBOOT_MAGIC1 0xfee1dead
-#define LINUX_REBOOT_MAGIC2 672274793
-#define LINUX_REBOOT_CMD_SW_SUSPEND 0xD000FCE2
-
-int main()
-{
- syscall(SYS_reboot, LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2,
- LINUX_REBOOT_CMD_SW_SUSPEND, 0);
- return 0;
-}
-
-Also /sys/ interface should be still present.
-
Q: What is 'suspend2'?
A: suspend2 is 'Software Suspend 2', a forked implementation of
should be held at that point and it must be safe to sleep there), and
add:
- if (current->flags & PF_FREEZE)
- refrigerator(PF_FREEZE);
+ try_to_freeze();
If the thread is needed for writing the image to storage, you should
-instead set the PF_NOFREEZE process flag when creating the thread.
+instead set the PF_NOFREEZE process flag when creating the thread (and
+be very careful).
-Q: What is the difference between between "platform", "shutdown" and
+Q: What is the difference between "platform", "shutdown" and
"firmware" in /sys/power/disk?
A:
"platform" is actually right thing to do, but "shutdown" is most
reliable.
+
+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 unneccessarily 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 `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" or "firmware" 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.
+
+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.
git://git.onelab.eu / linux-2.6.git / blobdiff