linux-kernel - Re: [RFC][PATCH 1/2 -mm] kexec based hibernation -v3: kexec jump

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Date:	Wed, 26 Sep 2007 16:30:36 -0400
From:	jfannin@...il.com (Joseph Fannin)
To:	Pavel Machek <pavel@....cz>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>, nigel@...pend2.net,
	"Huang, Ying" <ying.huang@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jeremy Maitin-Shepard <jbms@....edu>,
	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org,
	Kexec Mailing List <kexec@...ts.infradead.org>
Subject: Re: [RFC][PATCH 1/2 -mm] kexec based hibernation -v3: kexec jump

On Fri, Sep 21, 2007 at 11:45:12AM +0200, Pavel Machek wrote:
> Hi!
> > >
> > > Sounds doable, as long as you can cope with long command lines (which
> > > shouldn't be a biggie). (If you've got a swapfile or parts of a swap
> > > partition already in use, it can be quite fragmented).
> >
> > Hmm.  This is an interesting problem.  Sharing a swap file or a swap
> > partition with the actual swap of user space pages does seem to be
> > a limitation of this approach.
> >
> > Although the fact that it is simple to write to a separate file may
> > be a reasonable compensation.
>
> I'm not sure how you'd write it to a separate file. Notice that kjump
> kernel may not mount journalling filesystems, not even
> read-only. (Ext3 replays journal in that case). You could pass block
> numbers from the original kernel...

    The ext3 thing is a bug, the case for which I don't think has been
adequately explained to the ext[34] folks.  There should be at least a
no_replay mount flag available, or something.  It has ramifications
for more than just hibernation.

    And yeah, I'm gonna bring up the swap files thing again.  If you
can hibernate to a swap file, you can hibernate to a dedicated
hibernation file, and vice versa.

    If you can't hibernate to a swap file, then swap files are
effectively unsupported for any system you might want to hibernate.
<handwave> I wonder what embedded folks would think about that
</handwave>.

    But, in my ignorance, I'm not sure even fixing the ext3 bug will
guarantee you consistent metadata so that you can handle a
swap/hibernate file.  You can do a sync(), but how do you make that
not race against running processes without the freezer, or blkdev
snapshots?

    I guess uswsusp and the-patch-previously-known-as-suspend2 handle
this somehow, though.

   (It's that same ignorance that has me waiting for someone with
established credit with kernel people to make that argument for the
ext3 bug, so I can hang my own reasons for thinking that it's bad off
of theirs).

--
Joseph Fannin
jfannin@...il.com

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