[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 <rjw@sisk.pl>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 54 insertions, 0 deletions

diff --git a/Documentation/power/swsusp-and-swap-files.txt b/Documentation/power/swsusp-and-swap-files.txt
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+Using swap files with software suspend (swsusp)
+	(C) 2006 Rafael J. Wysocki <rjw@sisk.pl>
+
+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 <PAGE_SIZE>
+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=<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.