linux-kernel - Re: Back to the future.

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Date:	Fri, 27 Apr 2007 00:40:05 +0200
From:	Pavel Machek <pavel@....cz>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nigel Cunningham <nigel@...el.suspend2.net>,
	Pekka Enberg <penberg@...helsinki.fi>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Back to the future.

Hi!

> > * Doing things in the right order? (Prepare the image, then do the
> > atomic copy, then save).
> 
> I'd actually like to discuss this a bit..
> 
> I'm obviously not a huge fan of the whole user/kernel level split and 
> interfaces, but I actually do think that there is *one* split that makes 
> sense:
> 
>  - generate the (whole) snapshot image entirely inside the kernel
> 
>  - do nothing else (ie no IO at all), and just export it as a single image 
>    to user space (literally just mapping the pages into user space). 
>    *one* interface. None of the "pretty UI update" crap. Just a single 
>    system call:
> 
> 	void *snapshot_system(u32 *size);
> 
>    which will map in the snapshot, return the mapped address and the size 
>    (and if you want to support snapshots > 4GB, be my guest, but I suspect 
>    you're actually *better* off just admitting that if you cannot shrink 
>    the snapshot to less than 32 bits, it's not worth doing)

This is basically how uswsusp is designed. (We do not use system call,
you just read from /dev/snapshot, and you have to make few ioctls to
stop the other tasks).

> and for testing, you should be able to basically do
> 
> 	u32 size;
> 	void *buffer = snapshot_system(&size);
> 	if (buffer != MAP_FAILED)
> 		resume_snapshot(buffer, size);
> 
> and it should obviously work.

Which is what I did long time ago, during uswsusp development.

> Once you have that snapshot image in user space you can do anything you 
> want. And again: you'd hav a fully working system: not any degradation 
> *at*all*. If you're in X, then X will continue running etc even after the 
> snapshotting, although obviously the snapshotting will have tried to page 
> a lot of stuff out in order to make the snapshot smaller, so you'll likely 
> be crawling.

Well... We decided not to do this in the fully working system. SIGSTOP
is just not strong enough, and we want the snapshot atomic.

Now, it would be _very_ nice to be able to snapshot system and
continue running, but I just don't see how to do it without extensive
filesystem support.
									Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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