From kernel/suspend.c: * 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 change kernel command line between suspend and resume... * ...prepare for nasty fsck or worse. * * If you change your hardware while system is suspended... * ...well, it was not good idea. * * (*) suspend/resume support is needed to make it safe. You need to append resume=/dev/your_swap_partition to kernel command line. 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 Article about goals and implementation of Software Suspend for Linux ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Author: G‚ábor 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. 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. 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: 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: 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 #include #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 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: A kernel thread must voluntarily freeze itself (call 'refrigerator'). I found some kernel threads that don't do it, and they don't freeze so the system can't sleep. Is this a known behavior? A: All such kernel threads need to be fixed, one by one. Select the place where the thread is safe to be frozen (no kernel semaphores should be held at that point and it must be safe to sleep there), and add: 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 (and be very carefull). Q: What is the difference between between "platform", "shutdown" and "firmware" in /sys/power/disk? 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" firmware: tell bios to save state itself [needs BIOS-specific suspend partition, and has very little to do with swsusp] "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 managment, that's okay. But its 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 USB 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 heavilly, 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 usefull. 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: Why we cannot suspend to a swap file? A: Because accessing swap file needs the filesystem mounted, and filesystem might do something wrong (like replaying the journal) during mount. [Probably could be solved by modifying every filesystem to support some kind of "really read-only!" option. Patches welcome.]