Re: [PATCH 0/2] Kexec jump: The first step to kexec base hibernation

Huang, Ying
2007-07-14T10:57:19Z

On Sat, 2007-07-14 at 11:59 +0200, Rafael J. Wysocki wrote:
> > Hibernating process:
> >
> > 1. Normal kernel running
> > 2. Hibernating is triggered, sys_kexec_load is used to load
> > hibernating kernel and initramfs into memory. Then
> > sys_reboot(LINUX_REBOOT_CMD_KSPAWN) is invoked.
> > 3. In sys_reboot, kexec_jump is called to save device/CPU state,
> > then relocate_kernel is called. kexec_jump and relocate_kernel
> > reside in individual page in 16M~512M.
>
> OK
> What's going to happen to devices at this point?
>

The devices should be quiesced and the state of devices should be saved
in kexec_jump, before relocate_kernel is called. This needs the
implementation of device hibernating as you mentioned before.

> > 4. In relocate_kernel, 0~16M is backupped firstly, then the
> > hibernating kernel and initramfs is copied to 0~16M, after that,
> > the hibernating kernel is booted.
> > 5. In hibernating kernel, the memory of normal kernel (it is in
> > 16M~512M) is saved into a hibernation image through /dev/mem
> > and ELF header.
>
> I don't think it can be _that_ simple:
> (a) what about processes' memory
> (b) what about areas that shouldn't be saved?

The mem_map (struct page[]) of every zone of hibernated kernel is
checked. Necessary pages are saved, like memory snapshot of software
suspend, but in user space.

> > Resume process:
> >
> > 1. Resuming kernel is booted as a normal kernel, but the memory is
> > restricted to 0~16M.
> > 2. Checking whether there is a effective hibernation image. If
> > there isn't, the memory of 16M~512M is hot added, and the normal
> > boot up process continues; If there is, a resuming process is
> > triggered.
> > 3. sys_kexec_load is used to restore the memory state of hibernated
> > kernel. The sys_kexec_load works in crashdump way, that is, the
> > hibernation image is copied to destination location in 16M~512M
> > in sys_kexec_load instead of relocate_kernel. There is no half
> > of memory size restriction.
> > 4. sys_reboot is called to trigger jumping back, which will jump back
> > to kexec_jump of hibernated kernel.
> > 5. In kexec_jump of hibernated kernel, the memory of 0~16M is copied
> > back from the backup area in 16M~512M. The memory state of
> > hibernated kernel is restored totally. The CPU and device state
> > can be restored after that.
>
> Well, I don't know why this needs to be that complicated. We already have
> code in the mainline that's able to load a large hibernation image into memory
> and jump to the kernel being restored. And it has _no_ 50% of RAM limitation,
> this is the _saving_ part of the current code that this limitation comes from.

There is much similarity between sys_kexec_load and software resuming.
If resuming can be done by sys_kexec_load, then we need not two similar
functionality in kernel.

Best Regards,
Huang Ying