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Date: Wed, 26 Sep 2007 16:30:36 -0400
To: Pavel Machek <pavel@ucw.cz>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>, nigel@suspend2.net,
       "Huang, Ying" <ying.huang@intel.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Jeremy Maitin-Shepard <jbms@cmu.edu>, linux-kernel@vger.kernel.org,
       linux-pm@lists.linux-foundation.org,
       Kexec Mailing List <kexec@lists.infradead.org>
Subject: Re: [RFC][PATCH 1/2 -mm] kexec based hibernation -v3: kexec jump
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	"Huang, Ying" <ying.huang@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jeremy Maitin-Shepard <jbms@cmu.edu>, linux-kernel@vger.kernel.org,
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References: <1190266447.21818.17.camel@caritas-dev.intel.com> <200709211225.25874.nigel@nigel.suspend2.net> <1190342757.21818.75.camel@caritas-dev.intel.com> <200709211259.00195.nigel@nigel.suspend2.net> <m1r6ksiq27.fsf@ebiederm.dsl.xmission.com> <20070921094512.GA20149@elf.ucw.cz>
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From: jfannin@gmail.com (Joseph Fannin)
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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@gmail.com

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