linux-kernel - Re: Memory corruption during hibernation since 2.6.31

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Date:	Fri, 30 Jul 2010 09:01:46 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Ondrej Zary <linux@...nbow-software.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <balbir@...ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: Memory corruption during hibernation since 2.6.31

On Thu, 29 Jul 2010 11:44:31 -0700 (PDT)
Hugh Dickins <hughd@...gle.com> wrote:
 
> So, in between snapshotting the image and actually hibernating, we have
> two parallel universes, one frozen in the image, the other writing that
> out to swap: with the danger that the latter (which is about to die)
> will introduce fatal inconsistencies in the former by placing pages in
> swap locations innocently reallocated from it.  (Excuse me while I go
> write the movie script.)
> 
> I'd never got to think about that before.
> 
> Your fix is neat though hacky (it's somewhat of a coincidence that the
> usage arg happens to distinguish the hibernation case), and should be
> enough to fix "your" regression, but is it really enough?
> 
yes, it's hacky and this just hides a problem caused by the patch.

> I'm worrying about the try_to_free_swap() calls in shrink_page_list():
> can't those get called from direct reclaim (even if GFP_NOIO), and can't
> direct reclaim get invoked from somewhere in the I/O path below
> swap_writepage(), used for writing out the hibernation image?
> 
IIUC, before hibernation, shirnk_memory() is called and hibernation codes
expect there are enough memory. But you're right, there are no guarantee.

I think the best way is kexec(). But maybe rollback from hibernation failure
will be difficult. Considering how crash-dump works well and under maintainance
by many enterprise guys, hibernation-by-kexec is a choice. I think. It can make
reuse of kdump code, ...or, hibernation-resume code can eat kdump image
directly. Maybe the problem will be the speed of dump.

Otherwise, we need to take care of status changes in already-saved-memory.
Maybe anyone don't like to add COW method...

Thanks,
-Kame

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