On 2024-05-27 21:58, Christoph Hellwig wrote: > On Mon, May 27, 2024 at 12:51:07PM +0000, Sukrit.Bhatnagar@xxxxxxxx wrote: >> In my understanding, the resume offset in hibernate is used as follows. >> >> Suspend >> - Hibernate looks up the swap/swapfile using the details we pass in the >> sysfs entries, in the function swsusp_swap_check(): >> * /sys/power/resume - path/uuid/major:minor of the swap partition (or >> non-swap partition for swapfile) >> * /sys/power/resume_offset - physical offset of the swapfile in that >> partition >> * If no resume device is specified, it just uses the first available >> swap! - It then proceeds to write the image to the specified swap. >> (The allocation of swap pages is done by the swapfile code >> internally.) > > Where "it" is userspace code? If so, that already seems unsafe for > a swap device, but definitely is a no-go for a swapfile. By "it", I meant the hibernate code running in kernel space. Once userspace triggers hibernation by `echo disk > /sys/power/state` or a systemd wrapper program etc., and userspace tasks are frozen, everything happens within kernel context. >> - Hibernate gets the partition and offset values from kernel command-line >> parameters "resume" and "resume_offset" (which must be set from >> userspace, not ideal). > > Or is it just for these parameters? In which case we "only" need to > specify the swap file, which would then need code in the file system > driver to resolve the logical to physical mapping as swap files don't > need to be contiguous. Yes, it is just for setting these parameters in sysfs entries and in kernel commandline. I think specifying the swapfile path *may* not work because when we resume from hibernation, the filesystems are not yet mounted (except for the case when someone is resuming from initramfs stage). Using the block device + physical offset, this procedure becomes Independent of the filesystem and the mounted status. And since the system swap information is lost on reboot/shutdown, the kernel which loads the hibernation image will not know what swaps were enabled when the image was created. -- Sukrit
