From ecbd0da1eced957e0cbb611b4a4cb5b0cf63ba31 Mon Sep 17 00:00:00 2001 From: "Rafael J. Wysocki" Date: Wed, 6 Dec 2006 20:34:13 -0800 Subject: [PATCH] swsusp: document support for swap files Document the "resume_offset=" command line parameter as well as the way in which swap files are supported by swsusp. Signed-off-by: Rafael J. Wysocki Cc: Pavel Machek Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/power/swsusp-and-swap-files.txt | 54 +++++++++++++++++++++++++++ Documentation/power/swsusp.txt | 18 +++------ 2 files changed, 59 insertions(+), 13 deletions(-) create mode 100644 Documentation/power/swsusp-and-swap-files.txt (limited to 'Documentation/power') diff --git a/Documentation/power/swsusp-and-swap-files.txt b/Documentation/power/swsusp-and-swap-files.txt new file mode 100644 index 00000000000..e171d11dc65 --- /dev/null +++ b/Documentation/power/swsusp-and-swap-files.txt @@ -0,0 +1,54 @@ +Using swap files with software suspend (swsusp) + (C) 2006 Rafael J. Wysocki + +The Linux kernel handles swap files almost in the same way as it handles swap +partitions and there are only two differences between these two types of swap +areas: +(1) swap files need not be contiguous, +(2) the header of a swap file is not in the first block of the partition that +holds it. From the swsusp's point of view (1) is not a problem, because it is +already taken care of by the swap-handling code, but (2) has to be taken into +consideration. + +In principle the location of a swap file's header may be determined with the +help of appropriate filesystem driver. Unfortunately, however, it requires the +filesystem holding the swap file to be mounted, and if this filesystem is +journaled, it cannot be mounted during resume from disk. For this reason to +identify a swap file swsusp uses the name of the partition that holds the file +and the offset from the beginning of the partition at which the swap file's +header is located. For convenience, this offset is expressed in +units. + +In order to use a swap file with swsusp, you need to: + +1) Create the swap file and make it active, eg. + +# dd if=/dev/zero of= bs=1024 count= +# mkswap +# swapon + +2) Use an application that will bmap the swap file with the help of the +FIBMAP ioctl and determine the location of the file's swap header, as the +offset, in units, from the beginning of the partition which +holds the swap file. + +3) Add the following parameters to the kernel command line: + +resume= resume_offset= + +where is the partition on which the swap file is located +and is the offset of the swap header determined by the +application in 2). [Of course, this step may be carried out automatically +by the same application that determies the swap file's header offset using the +FIBMAP ioctl.] + +Now, swsusp will use the swap file in the same way in which it would use a swap +partition. [Of course this means that the resume from a swap file cannot be +initiated from whithin an initrd of initramfs image.] In particular, the +swap file has to be active (ie. be present in /proc/swaps) so that it can be +used for suspending. + +Note that if the swap file used for suspending is deleted and recreated, +the location of its header need not be the same as before. Thus every time +this happens the value of the "resume_offset=" kernel command line parameter +has to be updated. diff --git a/Documentation/power/swsusp.txt b/Documentation/power/swsusp.txt index e635e6f1e31..0761ff6c57e 100644 --- a/Documentation/power/swsusp.txt +++ b/Documentation/power/swsusp.txt @@ -297,20 +297,12 @@ 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? +Q: Can I 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. +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? -- cgit v1.2.3