linux-kernel - Re: Hibernating To Swap Considered Harmful

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Message-ID: <Pine.LNX.4.64.0707122250170.25614@asgard.lang.hm>
Date:	Thu, 12 Jul 2007 22:57:04 -0700 (PDT)
From:	david@...g.hm
To:	Joseph Fannin <jfannin@...il.com>
cc:	"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,
	Jeremy Maitin-Shepard <jbms@....edu>,
	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org
Subject: Re: Hibernating To Swap Considered Harmful

On Fri, 13 Jul 2007, Joseph Fannin wrote:

> Date: Fri, 13 Jul 2007 01:42:48 -0400
> On Thu, Jul 12, 2007 at 08:06:43PM -0700, david@...g.hm wrote:
>> On Thu, 12 Jul 2007, Rafael J. Wysocki wrote:
>
>>> Plus we need to figure out how to avoid corrupting filesystems and
>>> swap in use by the "old" kernel and its processes (hint: a separate
>>> "hibernation partition" is a no-go).
>>
>> I thought the existing hibernation wrote to the swap partition as it's
>> dedicated space?
>>
>> I didn't know that anyone was suggesting writing the hibernation image to
>> a filesystem that the kernel was activly accessing.
>
> I'm suggesting a dedicated, preallocated hibernation *file*, right
> now.  There's no way around it, if hibernation is to be reliable --
> otherwise hibernation can fail if the system has used enough of its
> swap space, so that there isn't enough room to write the hibernate
> image.
>
> Even if it's desirable to allow hibernation to fail if the system is
> too deep into swap, it's a moot point.
>
> Consider how the need to ensure that there is enough space to write
> the hibernate image is dealt with now:  by making a big honking swap
> space, so big that enough of it is all but guaranteed to be free,
> except under the heaviest of memory usage.  So the space is already
> reserved -- and now that it's commingled with actual swap, you have the
> need to pass the swap data structures between the two kernels.
>
> Consider instead,  you set up two swap spaces, one regular, and one
> for hibernation. You don't touch the "hibernation swap" unless the
> other is full -- I think just setting a lower priority on the swap
> space is enough for this.  Before you jump to the hibernate kernel,
> you swapoff that hibernate swap.
>
> If you can't swapoff the hibernate swap, hibernate fails right there.
>
> If you can, you have your space for writing the image, free and clear
> of any of the original kernel's internal state.  There isn't any need
> to treat that space as swap any more at all -- the only reason to do
> so would be to reuse the existing code.
>
> Setting aside two partitions for swap is obviously undesireable, but
> thankfully, Linux supports swap *files*.

the only justification I have heard for why the hibernate image must be 
written to the swap partition is backwards compatibility (i.e., we've 
always done it that way)

if you are going to reserve disk space for hibernation, what is so bad 
about useing a normal partition?

David Lang
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