[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>
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@ -1370,6 +1370,12 @@ and is between 256 and 4096 characters. It is defined in the file
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resume= [SWSUSP]
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resume= [SWSUSP]
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Specify the partition device for software suspend
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Specify the partition device for software suspend
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resume_offset= [SWSUSP]
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Specify the offset from the beginning of the partition
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given by "resume=" at which the swap header is located,
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in <PAGE_SIZE> units (needed only for swap files).
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See Documentation/power/swsusp-and-swap-files.txt
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rhash_entries= [KNL,NET]
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rhash_entries= [KNL,NET]
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Set number of hash buckets for route cache
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Set number of hash buckets for route cache
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54
Documentation/power/swsusp-and-swap-files.txt
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54
Documentation/power/swsusp-and-swap-files.txt
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Using swap files with software suspend (swsusp)
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(C) 2006 Rafael J. Wysocki <rjw@sisk.pl>
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The Linux kernel handles swap files almost in the same way as it handles swap
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partitions and there are only two differences between these two types of swap
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areas:
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(1) swap files need not be contiguous,
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(2) the header of a swap file is not in the first block of the partition that
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holds it. From the swsusp's point of view (1) is not a problem, because it is
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already taken care of by the swap-handling code, but (2) has to be taken into
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consideration.
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In principle the location of a swap file's header may be determined with the
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help of appropriate filesystem driver. Unfortunately, however, it requires the
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filesystem holding the swap file to be mounted, and if this filesystem is
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journaled, it cannot be mounted during resume from disk. For this reason to
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identify a swap file swsusp uses the name of the partition that holds the file
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and the offset from the beginning of the partition at which the swap file's
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header is located. For convenience, this offset is expressed in <PAGE_SIZE>
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units.
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In order to use a swap file with swsusp, you need to:
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1) Create the swap file and make it active, eg.
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# dd if=/dev/zero of=<swap_file_path> bs=1024 count=<swap_file_size_in_k>
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# mkswap <swap_file_path>
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# swapon <swap_file_path>
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2) Use an application that will bmap the swap file with the help of the
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FIBMAP ioctl and determine the location of the file's swap header, as the
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offset, in <PAGE_SIZE> units, from the beginning of the partition which
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holds the swap file.
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3) Add the following parameters to the kernel command line:
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resume=<swap_file_partition> resume_offset=<swap_file_offset>
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where <swap_file_partition> is the partition on which the swap file is located
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and <swap_file_offset> is the offset of the swap header determined by the
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application in 2). [Of course, this step may be carried out automatically
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by the same application that determies the swap file's header offset using the
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FIBMAP ioctl.]
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Now, swsusp will use the swap file in the same way in which it would use a swap
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partition. [Of course this means that the resume from a swap file cannot be
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initiated from whithin an initrd of initramfs image.] In particular, the
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swap file has to be active (ie. be present in /proc/swaps) so that it can be
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used for suspending.
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Note that if the swap file used for suspending is deleted and recreated,
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the location of its header need not be the same as before. Thus every time
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this happens the value of the "resume_offset=" kernel command line parameter
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has to be updated.
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@ -297,20 +297,12 @@ system is shut down or suspended. Additionally use the encrypted
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suspend image to prevent sensitive data from being stolen after
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suspend image to prevent sensitive data from being stolen after
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resume.
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resume.
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Q: Why can't we suspend to a swap file?
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Q: Can I suspend to a swap file?
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A: Because accessing swap file needs the filesystem mounted, and
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A: Generally, yes, you can. However, it requires you to use the "resume=" and
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filesystem might do something wrong (like replaying the journal)
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"resume_offset=" kernel command line parameters, so the resume from a swap file
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during mount.
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cannot be initiated from an initrd or initramfs image. See
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swsusp-and-swap-files.txt for details.
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There are few ways to get that fixed:
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1) Probably could be solved by modifying every filesystem to support
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some kind of "really read-only!" option. Patches welcome.
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2) suspend2 gets around that by storing absolute positions in on-disk
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image (and blocksize), with resume parameter pointing directly to
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suspend header.
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Q: Is there a maximum system RAM size that is supported by swsusp?
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Q: Is there a maximum system RAM size that is supported by swsusp?
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