X-Git-Url: http://git.onelab.eu/?a=blobdiff_plain;f=Documentation%2Fpower%2Fswsusp.txt;h=d7814a113ee1752a7c91c54ae6841407baa73370;hb=43bc926fffd92024b46cafaf7350d669ba9ca884;hp=cbbbc9c0764f0ac02f2ef179c5ec48394ef03bbd;hpb=c7b5ebbddf7bcd3651947760f423e3783bbe6573;p=linux-2.6.git

diff --git a/Documentation/power/swsusp.txt b/Documentation/power/swsusp.txt
index cbbbc9c07..d7814a113 100644
--- a/Documentation/power/swsusp.txt
+++ b/Documentation/power/swsusp.txt
@@ -1,24 +1,49 @@
-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 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.
  *
- * If you change kernel command line between suspend and resume...
- *			        ...prepare for nasty fsck or worse.
+ * (*) suspend/resume support is needed to make it safe.
  *
- * (*) pm interface support is needed to make it safe.
+ * If you have any filesystems on USB devices mounted before 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).
 
 You need to append resume=/dev/your_swap_partition to kernel command
-line. Then you suspend by echo 4 > /proc/acpi/sleep.
+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 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
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -32,42 +57,24 @@ 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
-real high when running from batteries. The other gain is that we don't have to
+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.
 
-Using the code
-
-You have two ways to use this code. The first one is is with a patched
-SysVinit (my patch is against 2.76 and available at my home page). You
-might call 'swsusp' or 'shutdown -z <time>'. Next way is to echo 4 >
-/proc/acpi/sleep.
-
-Either way it saves the state of the machine into active swaps and then
-reboots.  You must explicitly specify the swap partition to resume from with
+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 touch any of the
-hardware!
+In the meantime while the system is suspended you should not add/remove any
+of the hardware, write to the filesystems, etc.
 
-About the code
+Sleep states summary
+====================
 
-Things to implement
-- We should only make a copy of data related to kernel segment, since any
-  process data won't be changed.
-- Should make more sanity checks. Or are these enough?
-
-Not so important ideas for implementing
-
-- If a real time process is running then don't suspend the machine.
-- Support for adding/removing hardware while suspended?
-- We should not free pages at the beginning so aggressively, most of them
-  go there anyway..
-
-Sleep states summary (thanx, Ducrot)
-====================================
+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
@@ -79,7 +86,6 @@ 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
 ==========================
 
@@ -93,11 +99,6 @@ 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?
 
@@ -123,44 +124,7 @@ advance. That means that the limitation goes away in practice.
 
 Q: Does linux support ACPI S4?
 
-A: No.
-
-When swsusp was created, ACPI was not too widespread, so we tried to
-avoid using ACPI-specific stuff. ACPI also is/was notoriously
-buggy. These days swsusp works on APM-only i386 machines and even
-without any power managment at all. Some versions also work on PPC.
-
-That means that machine does not enter S4 on suspend-to-disk, but
-simply enters S5. That has few advantages, you can for example boot
-windows on next boot, and return to your Linux session later. You
-could even have few different Linuxes on your box (not sharing any
-partitions), and switch between them.
-
-It also has disadvantages. On HP nx5000, if you unplug power cord
-while machine is suspended-to-disk, Linux will fail to notice that.
-
-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;
-}
+A: Yes. That's what echo platform > /sys/power/disk does.
 
 Q: What is 'suspend2'?
 
@@ -175,16 +139,21 @@ 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: Kernel thread must voluntarily freeze itself (call 'refrigerator'). But
-I found some kernel threads don't do it, and they don't freeze, and
+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 place
-where it is safe to be frozen (no kernel semaphores should be held at
-that point and it must be safe to sleep there), and add:
+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).
 
-            if (current->flags & PF_FREEZE)
-                    refrigerator(PF_FREEZE);
 
 Q: What is the difference between between "platform", "shutdown" and
 "firmware" in /sys/power/disk?
@@ -201,3 +170,204 @@ firmware: tell bios to save state itself [needs BIOS-specific suspend
 
 "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 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: Why can't we 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.
+
+There are few ways to get that fixed:
+
+1) Probably could be solved by modifying every filesystem to support
+some kind of "really read-only!" option. Patches welcome.
+
+2) suspend2 gets around that by storing absolute positions in on-disk
+image (and blocksize), with resume parameter pointing directly to
+suspend header.
+
+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 5, for example by doing
+
+	echo 5 > /proc/sys/kernel/printk
+
+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.  It depends on your hardware, and it could be true even for
+suspend-to-RAM.  In fact, even with "-o sync" you can lose data if your
+programs have information in buffers they haven't written out to disk.
+
+If you're lucky, your hardware will support low-power modes for USB
+controllers while the system is asleep.  Lots of hardware doesn't,
+however.  Shutting off the power to a USB controller is equivalent to
+unplugging all the attached devices.
+
+Remember that it's always a bad idea to unplug a disk drive containing a
+mounted filesystem.  With USB that's true even when your system is asleep!
+The safest thing is to unmount all USB-based filesystems before suspending
+and remount them after resuming.
+