Re: [lkcd-devel] Re: What's left over.

[ebiederm@xmission.com (Eric W. Biederman)]

Linus Torvalds <torvalds@transmeta.com> writes:

> >From a sanity standpoint, I think the thing already _has_ a system call, 
> though: clearly "sys_reboot()" is the place to add a case for "reboot into 
> this image". No? That's where we shut down devices anyway, and it's the 
> sane place to say "reboot into the kexec image"

When kexec is separated into two pieces I agree.  As I had it
initially in one step it does not look at all like reboot.    Now I
just need to think up a new magic number for sys_reboot.

[snip wonderful vision of the theoretical simplicity of sys_kexec].

In case I was not sufficiently clear last night.  It could be as
simple as your example code if I replaced vmalloc by
__get_free_pages/alloc_pages, and allocated a large contiguous area of
ram.  But MAX_ORDER limits me to 8MB images, and allocating an 8MB
chunk is unreliable, and even a 2MB chunk is dangerous.    

So I must use some form of scatter/gather list of pages, like
area ->pages[] to make it work.  Things get tricky because I gather
(via memcpy) the pages at a location that potentially overlaps the
source pages.  So I must walk through the list of pages making certain
I when I gather (memcpy) the buffer pages into their final location I
will not stomp on a buffer page I have not come to yet. Correctly
doing that untangling is where the complexity in kernel/kexec.c comes
from.

Eric