linux-kernel - Re: [RFC][PATCH] PM / Sleep: Freeze filesystems during system suspend/hibernation

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Date:	Fri, 10 Feb 2012 02:52:17 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Pavel Machek <pavel@....cz>, "Rafael J. Wysocki" <rjw@...k.pl>,
	Linux PM list <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>,
	linux-fsdevel@...r.kernel.org, Dave Chinner <david@...morbit.com>,
	Nigel Cunningham <nigel@...onice.net>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH] PM / Sleep: Freeze filesystems during system
 suspend/hibernation

Alan Stern wrote:
> On Wed, 1 Feb 2012, Pavel Machek wrote:
> 
> > > particular, this should help to solve a long-standing issue that, in
> > > some cases, during resume from hibernation the boot loader causes the
> > > journal to be replied for the filesystem containing the kernel image
> > > and/or initrd causing it to become inconsistent with the information
> > > stored in the hibernation image.
> > 
> > Ungood. Why is bootloader/initrd doing that? If it mounts filesystem
> > read/write, what is the guarantee that it will not change data on the
> > filesystem, breaking stuff?
> > 
> > Bootloaders should just not replay journals.
> > 
> > > The user-space-driven hibernation (s2disk) is not covered by this
> > > change, because the freezing of filesystems prevents s2disk from
> > > accessing device special files it needs to do its job.
> > 
> > ...so bootloaders need to be fixed, anyway.
> 
> I don't know about bootloaders, but from what I've heard, Linux fs
> drivers (including those inside initrds) always replay the journal,
> even if the filesystem is mounted read-only.  This could be considered
> a bug in the filesystem code.

Theoretically a filesystem might need replay for the bootloader to see
a non-corrupt image, even for just the files it uses.

For example if the last state was in the middle of updating the root
directory, the /boot entry in the root directory might not be reliably
found without replaying the journal.

However replaying a journal when mounted read-only should probably
track journalled blocks in memory only, not commit back to the storage.

All the best,
-- Jamie
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