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

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Date:	Sat, 14 Jul 2007 00:51:13 -0700 (PDT)
From:	david@...g.hm
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Jeremy Maitin-Shepard <jbms@....edu>,
	"Huang, Ying" <ying.huang@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Machek <pavel@....cz>, nigel@...el.suspend2.net,
	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org
Subject: Re: [PATCH 0/2] Kexec jump: The first step to kexec base hibernation

On Fri, 13 Jul 2007, Rafael J. Wysocki wrote:

>> Ok, now we need a data channel from the old kernel to the hibernate
>> kernel, to the restore kernel. and the messier the memory layout the
>> larger this data channel needs to be (hmm, what's the status on the memory
>> defrag patches being proposed?) if this list can be made small enough it
>> would work to just have the old kernel put the data in a known location in
>> ram, and let the other two parts find it (in ram for the hibernate kernel,
>> in the hibernate image for the wakeup kernel).
>
> I think the hibernation kernel should mmap() the "old" kernel's (and it's
> processes') memory available for saving, so that the image-saving process
> can read its contents from the original locations.

but I'll bet that not all kernels keep the info in the same place (and 
probably not even in the same format). I'm suggesting that a standard be 
defined for the format of the data and the location of a pointer to it 
that will be maintained across kernel versions.

>> how do the existing hibernate processes store this?
>
> There are two approaches.  In the first of them (used in the mainline) we just
> create copies of all pages eligible for saving (hence we can't create images
> larger than 50% of RAM) atomically and then we save the contents of these
> copies (either directly from the kernel or through a user space process).  This
> way we don't need to worry that they may be modified before we can save
> them.
>
> The other approach is the Nigel's one, in which all LRU pages are first saved
> and then used as additional storage for copying the rest of memory contents.
> This has a drawback that we are not 100% sure if the LRU won't be modified
> after we've used them to store the copies of the other pages.

and since both current approaches use the same kernel for everything the 
issues I'm thinking of simply don't apply.

>> since people are complaining about the amount of ram that a kexec kernel
>> would take up I'm assuiming it's somethingmore complex then just a bitmap
>> of all possible pages.
>
> No, it's just bitmaps, AFAICS, and the complaints are a bit overstated, IMO. ;-)

1 bit for each 4k means 1m bits for 4g of ram, or 128k of bitmaps, growing 
up to 1m of ram used for 32G of ram in the system. I guess this isn't bad 
as long as it doesn't need to be contiguous for the new kernel to access 
it.

ok, that makes it a pretty trivial thing to work with. I just need to 
learn how to find the bitmaps.

>> most of the conversation so far has been around the process of makeing the
>> snapshot and storing it. what are the processes and tools available to
>> restore images?
>
> We have quite an efficient restoration code in the kernel right now.  It's
> able to upload big images (something like total RAM minus the size of the
> boot kernel, initrd and, optionally, the resume application), which is much
> more than we're able to save. :-)
>
> It can work with images uploaded via /dev/snapshot from the user space
> (specific image format is required, but that can be changed easily).

Ok, so it sounds as if the restore is basicly a solved problem, great!

so now I want to do a but-ugly hibernate configuration.

while I haven't done it yet I am very confident that I can enable kexec 
and create a kernel and filesystem to boot into. I can then use perl to 
get the bitmaps out of kmem (once I learn how to find them), and then can 
read from kmem to a file to save the 4k chunks that I need to a file.

it'll be ugly and use lots of partitions, but it should end up being solid 
from what I am understanding.

where can I learn how to find the bitmaps and what format things need to 
be in to feed into /dev/snapshot?

David Lang
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