[PATCH -mm 5/5] swsusp: Document support for swap files (rev. 2)

From: Rafael J. Wysocki
Date: Wed Sep 27 2006 - 02:04:07 EST

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 <rjw@xxxxxxx>
Documentation/kernel-parameters.txt | 6 ++
Documentation/power/swsusp-and-swap-files.txt | 54 ++++++++++++++++++++++++++
Documentation/power/swsusp.txt | 18 ++------
3 files changed, 65 insertions(+), 13 deletions(-)

Index: linux-2.6.18-mm1/Documentation/kernel-parameters.txt
--- linux-2.6.18-mm1.orig/Documentation/kernel-parameters.txt
+++ linux-2.6.18-mm1/Documentation/kernel-parameters.txt
@@ -1368,6 +1368,12 @@ and is between 256 and 4096 characters.
resume= [SWSUSP]
Specify the partition device for software suspend

+ resume_offset= [SWSUSP]
+ Specify the offset from the beginning of the partition
+ given by "resume=" at which the swap header is located,
+ in <PAGE_SIZE> units (needed only for swap files).
+ See Documentation/power/swsusp-and-swap-files.txt
rhash_entries= [KNL,NET]
Set number of hash buckets for route cache

Index: linux-2.6.18-mm1/Documentation/power/swsusp-and-swap-files.txt
--- /dev/null
+++ linux-2.6.18-mm1/Documentation/power/swsusp-and-swap-files.txt
@@ -0,0 +1,54 @@
+Using swap files with software suspend (swsusp)
+ (C) 2006 Rafael J. Wysocki <rjw@xxxxxxx>
+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
+(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
+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 <PAGE_SIZE>
+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=<swap_file_path> bs=1024 count=<swap_file_size_in_k>
+# mkswap <swap_file_path>
+# swapon <swap_file_path>
+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 <PAGE_SIZE> units, from the beginning of the partition which
+holds the swap file.
+3) Add the following parameters to the kernel command line:
+resume=<swap_file_partition> resume_offset=<swap_file_offset>
+where <swap_file_partition> is the partition on which the swap file is located
+and <swap_file_offset> 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.
Index: linux-2.6.18-mm1/Documentation/power/swsusp.txt
--- linux-2.6.18-mm1.orig/Documentation/power/swsusp.txt
+++ linux-2.6.18-mm1/Documentation/power/swsusp.txt
@@ -297,20 +297,12 @@ system is shut down or suspended. Additi
suspend image to prevent sensitive data from being stolen after

-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?

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