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authorRafael J. Wysocki <rjw@sisk.pl>2006-12-06 23:34:13 -0500
committerLinus Torvalds <torvalds@woody.osdl.org>2006-12-07 11:39:27 -0500
commitecbd0da1eced957e0cbb611b4a4cb5b0cf63ba31 (patch)
treec7756453fa284eb1cf27b914be9ea9a8303b87d4 /Documentation/power
parent9a154d9d95b7b9845938242f5c62505b3cab5bcd (diff)
[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>
Diffstat (limited to 'Documentation/power')
-rw-r--r--Documentation/power/swsusp-and-swap-files.txt54
-rw-r--r--Documentation/power/swsusp.txt18
2 files changed, 59 insertions, 13 deletions
diff --git a/Documentation/power/swsusp-and-swap-files.txt b/Documentation/power/swsusp-and-swap-files.txt
new file mode 100644
index 000000000000..e171d11dc656
--- /dev/null
+++ b/Documentation/power/swsusp-and-swap-files.txt
@@ -0,0 +1,54 @@
1Using swap files with software suspend (swsusp)
2 (C) 2006 Rafael J. Wysocki <rjw@sisk.pl>
3
4The Linux kernel handles swap files almost in the same way as it handles swap
5partitions and there are only two differences between these two types of swap
6areas:
7(1) swap files need not be contiguous,
8(2) the header of a swap file is not in the first block of the partition that
9holds it. From the swsusp's point of view (1) is not a problem, because it is
10already taken care of by the swap-handling code, but (2) has to be taken into
11consideration.
12
13In principle the location of a swap file's header may be determined with the
14help of appropriate filesystem driver. Unfortunately, however, it requires the
15filesystem holding the swap file to be mounted, and if this filesystem is
16journaled, it cannot be mounted during resume from disk. For this reason to
17identify a swap file swsusp uses the name of the partition that holds the file
18and the offset from the beginning of the partition at which the swap file's
19header is located. For convenience, this offset is expressed in <PAGE_SIZE>
20units.
21
22In order to use a swap file with swsusp, you need to:
23
241) Create the swap file and make it active, eg.
25
26# dd if=/dev/zero of=<swap_file_path> bs=1024 count=<swap_file_size_in_k>
27# mkswap <swap_file_path>
28# swapon <swap_file_path>
29
302) Use an application that will bmap the swap file with the help of the
31FIBMAP ioctl and determine the location of the file's swap header, as the
32offset, in <PAGE_SIZE> units, from the beginning of the partition which
33holds the swap file.
34
353) Add the following parameters to the kernel command line:
36
37resume=<swap_file_partition> resume_offset=<swap_file_offset>
38
39where <swap_file_partition> is the partition on which the swap file is located
40and <swap_file_offset> is the offset of the swap header determined by the
41application in 2). [Of course, this step may be carried out automatically
42by the same application that determies the swap file's header offset using the
43FIBMAP ioctl.]
44
45Now, swsusp will use the swap file in the same way in which it would use a swap
46partition. [Of course this means that the resume from a swap file cannot be
47initiated from whithin an initrd of initramfs image.] In particular, the
48swap file has to be active (ie. be present in /proc/swaps) so that it can be
49used for suspending.
50
51Note that if the swap file used for suspending is deleted and recreated,
52the location of its header need not be the same as before. Thus every time
53this happens the value of the "resume_offset=" kernel command line parameter
54has to be updated.
diff --git a/Documentation/power/swsusp.txt b/Documentation/power/swsusp.txt
index e635e6f1e316..0761ff6c57ed 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
297suspend image to prevent sensitive data from being stolen after 297suspend image to prevent sensitive data from being stolen after
298resume. 298resume.
299 299
300Q: Why can't we suspend to a swap file? 300Q: Can I suspend to a swap file?
301 301
302A: Because accessing swap file needs the filesystem mounted, and 302A: Generally, yes, you can. However, it requires you to use the "resume=" and
303filesystem might do something wrong (like replaying the journal) 303"resume_offset=" kernel command line parameters, so the resume from a swap file
304during mount. 304cannot be initiated from an initrd or initramfs image. See
305 305swsusp-and-swap-files.txt for details.
306There are few ways to get that fixed:
307
3081) Probably could be solved by modifying every filesystem to support
309some kind of "really read-only!" option. Patches welcome.
310
3112) suspend2 gets around that by storing absolute positions in on-disk
312image (and blocksize), with resume parameter pointing directly to
313suspend header.
314 306
315Q: Is there a maximum system RAM size that is supported by swsusp? 307Q: Is there a maximum system RAM size that is supported by swsusp?
316 308