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

[Huang, Ying]

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
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