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1===============================================2Using swap files with software suspend (swsusp)3===============================================4 5 (C) 2006 Rafael J. Wysocki <rjw@sisk.pl>6 7The Linux kernel handles swap files almost in the same way as it handles swap8partitions and there are only two differences between these two types of swap9areas:10(1) swap files need not be contiguous,11(2) the header of a swap file is not in the first block of the partition that12holds it. From the swsusp's point of view (1) is not a problem, because it is13already taken care of by the swap-handling code, but (2) has to be taken into14consideration.15 16In principle the location of a swap file's header may be determined with the17help of appropriate filesystem driver. Unfortunately, however, it requires the18filesystem holding the swap file to be mounted, and if this filesystem is19journaled, it cannot be mounted during resume from disk. For this reason to20identify a swap file swsusp uses the name of the partition that holds the file21and the offset from the beginning of the partition at which the swap file's22header is located. For convenience, this offset is expressed in <PAGE_SIZE>23units.24 25In order to use a swap file with swsusp, you need to:26 271) Create the swap file and make it active, eg.::28 29 # dd if=/dev/zero of=<swap_file_path> bs=1024 count=<swap_file_size_in_k>30 # mkswap <swap_file_path>31 # swapon <swap_file_path>32 332) Use an application that will bmap the swap file with the help of the34FIBMAP ioctl and determine the location of the file's swap header, as the35offset, in <PAGE_SIZE> units, from the beginning of the partition which36holds the swap file.37 383) Add the following parameters to the kernel command line::39 40 resume=<swap_file_partition> resume_offset=<swap_file_offset>41 42where <swap_file_partition> is the partition on which the swap file is located43and <swap_file_offset> is the offset of the swap header determined by the44application in 2) (of course, this step may be carried out automatically45by the same application that determines the swap file's header offset using the46FIBMAP ioctl)47 48OR49 50Use a userland suspend application that will set the partition and offset51with the help of the SNAPSHOT_SET_SWAP_AREA ioctl described in52Documentation/power/userland-swsusp.rst (this is the only method to suspend53to a swap file allowing the resume to be initiated from an initrd or initramfs54image).55 56Now, swsusp will use the swap file in the same way in which it would use a swap57partition. In particular, the swap file has to be active (ie. be present in58/proc/swaps) so that it can be used for suspending.59 60Note that if the swap file used for suspending is deleted and recreated,61the location of its header need not be the same as before. Thus every time62this happens the value of the "resume_offset=" kernel command line parameter63has to be updated.64