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Subject: Re: [lkcd-devel] Re: What's left over.
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Linus Torvalds <torvalds@transmeta.com>,
       Werner Almesberger <wa@almesberger.net>,
       Suparna Bhattacharya <suparna@in.ibm.com>,
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On Thu, 2002-11-07 at 08:50, Eric W. Biederman wrote:
> panic does not call sys_reboot it rolls that functionality by hand.
> And to a certain extent it makes sense for panic to take a different
> path because we know something is badly wrong so we need to be extra
> careful.

However both of them should use the same end point routines and the
hooks should go there

> reboots or panics.   This memory is not accounted for anywhere so no
> one can see that we have it allocated, which makes debugging hard.
> Additionally in locking up megabytes for a long period of time we
> create unsolvable fragmentation issues for the mm layer to deal with.

We have an MMU so if you just n thousand "get me a page" calls its quite
happy.

> In a unified design I can buffer the image in the anonymous pages of a
> user space process just as well as I can in locked down kernel memory.
> So factoring sys_kexec in to load and execute pieces only helps for
> executing the new image on a kernel panic, and that case does not
> actually work.

What if your user space is swapped out - you can't page it back in
safely

> - How should the pages allocated to an early loaded image be accounted
>   for?

Just get_free_page them - if you can handle it over 4Gb then specify
that high pages are fine and kmap them to copy into them - that makes
the VM on giant boxes way happier. For bonus points also adjust the
virtual memory accounting.

> - How do we avoid making life hard for the mm system with an early
>   loaded image?

Not really, especially if you are allowing high pages

> - Is it safe to call sys_reboot from panic?

No but both can call sys_machine_restart or whatever

> - Can we simply factor out the sequence:
> 		notifier_call_chain(&reboot_notifier_list, SYS_RESTART, NULL);
> 		system_running = 0;
> 		device_shutdown();
>   And place it into it's own subroutine?

Don't do that sequence on a panic IMHO (this is a standing issue, we
should not pass NULL but REBOOT/PANIC/KEXEC/... so the drivers can make
that decision - then we can do it right

Alan

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