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Message-Id: <200609231213.02501.rjw@sisk.pl> Date: Sat, 23 Sep 2006 12:13:01 +0200 From: "Rafael J. Wysocki" <rjw@...k.pl> To: Andrew Morton <akpm@...l.org> Cc: Dave Jones <davej@...hat.com>, Pavel Machek <pavel@....cz>, LKML <linux-kernel@...r.kernel.org> Subject: [PATCH -mm 6/6] 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@...k.pl> Acked-by: Pavel Machek <pavel@....cz> --- Documentation/kernel-parameters.txt | 6 ++ Documentation/power/swsusp-and-swap-files.txt | 60 ++++++++++++++++++++++++++ Documentation/power/swsusp.txt | 18 ++----- 3 files changed, 71 insertions(+), 13 deletions(-) Index: linux-2.6.18-rc7-mm1/Documentation/kernel-parameters.txt =================================================================== --- linux-2.6.18-rc7-mm1.orig/Documentation/kernel-parameters.txt +++ linux-2.6.18-rc7-mm1/Documentation/kernel-parameters.txt @@ -1370,6 +1370,12 @@ 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-rc7-mm1/Documentation/power/swsusp-and-swap-files.txt =================================================================== --- /dev/null +++ linux-2.6.18-rc7-mm1/Documentation/power/swsusp-and-swap-files.txt @@ -0,0 +1,60 @@ +Using swap files with software suspend (swsusp) + (C) 2006 Rafael J. Wysocki <rjw@...k.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> + +Then, the kernel will place the following line in the dmesg: + +Adding <swap_file_size_in_k - 1>k swap on <swap_file_path>. Priority:-2 extents:<nr_extents> across:<span_in_k>k offset:<swap_file_offset> + +where <nr_extents>, <span_in_k>, <swap_file_offset> are numbers that describe +the swap file configuration. The last one, <swap_file_offset>, is needed by +swsusp. + +2) 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. + +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 the location of a swap file's header may also be determined by +an application using the FIBMAP ioctl to retrieve the header's block number +from the kernel. Next, the information obtained this way can be used to +update the kernel command line in the boot loader configuration file. +This approach may be used, for example, by distribution installers and +automated administration tools. + +Note also 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-rc7-mm1/Documentation/power/swsusp.txt =================================================================== --- linux-2.6.18-rc7-mm1.orig/Documentation/power/swsusp.txt +++ linux-2.6.18-rc7-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 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? - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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