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diff --git a/kernel/Documentation/power/swsusp.txt b/kernel/Documentation/power/swsusp.txt new file mode 100644 index 000000000..f732a8321 --- /dev/null +++ b/kernel/Documentation/power/swsusp.txt @@ -0,0 +1,429 @@ +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.) + +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 + +. 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 500 MB 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/usb/persist.txt. + +Q: Can I suspend-to-disk using a swap partition under LVM? + +A: No. You can suspend successfully, but you'll not be able to +resume. uswsusp should be able to work with LVM. See suspend.sf.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. |