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SubjectRe: Hibernating To Swap Considered Harmful
On Tue, 17 Jul 2007, Joseph Fannin wrote:

> On Mon, Jul 16, 2007 at 11:42:08PM -0700, 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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