linux-kernel - Re: Hibernating To Swap Considered Harmful

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Date:	Tue, 17 Jul 2007 00:34:16 -0700 (PDT)
From:	david@...g.hm
To:	Joseph Fannin <jfannin@...il.com>
cc:	Oliver Neukum <oliver@...kum.org>,
	Jeremy Maitin-Shepard <jbms@....edu>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"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: Hibernating To Swap Considered Harmful

On Tue, 17 Jul 2007, Joseph Fannin wrote:

> On Mon, Jul 16, 2007 at 11:42:08PM -0700, david@...g.hm wrote:
>> On Tue, 17 Jul 2007, Joseph Fannin wrote:
>
>>> root is free to "dd if=/dev/random of=/dev/mem".  Root owned
>>> daemons which do bad things are bugs.
>>
>> in this case it would be more like
>>
>> dd if=/block0 of=/dev/sda1 count=1 bs=4096 skip=5000
>> dd if=/block1 of=/dev/sda1 count=1 bs=4096 skip=5050
>> dd if=/block2 of=/dev/sda1 count=1 bs=4096 skip=5400
>> etc
>>
>> to write the blocks to the raw parition in the right place
>
> What I meant by that was that root is allowed to shoot himself in the
> foot.  Nothing stops root from opening a swap/hibernate file, which
> would put it in cache, and cause it to be inconsistant if a
> hibernation image was written to it behind the kernel's back.
>
> That would be a very stupid thing to do, however.  There's no reason
> to open that file, unless you know *exactly* what you are doing, in
> which case the onus is on you to get it right.
>
> But you have a point.  The swap file could be very fragmented.  It
> might often be so, even.
>
> Still, is this better than exporting the kernel's swap internals
> (which has never been a public interface before)?
>
> Does it make the interface that writing hibernation images to swap
> imposes any better?
>
> Even if hibernation files are no less complicated to support than
> hibernating to swap files (which isn't a forgone conclusion) , there
> are plenty of reasons writing hibernation images to swap doesn't make
> sense.
>
>
>>> Again, supporting swap files (*which is not optional*) requires the
>>> very same support.
>>
>> in the kexec model why would the second kernel care about swap files at
>> all? (unles it chooses to write to them, in which case it is exactly the
>> same support, but unless it writes to them it doesn't need to care)
>
> My point is that no extra work is required to write hibernation images
> to dedicated files than to write hibernation images to swap files.
>
> If swap files are to be supported, then, there's no technical reason
> not to support dedicated hibernation files.  Dedicated hibernation
> files are better, and there's no reason not to implement them.

I agree with your point, but the reverse is not true, the ability to write 
to a dedicated hibernation file does not produce the capacity to write to 
a swap file, and I do question the 'requirement' to write the hibernation 
image to the swap file.

David Lang

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