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

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Message-ID: <20131221233318.GA7126@amd.pavel.ucw.cz>
Date:	Sun, 22 Dec 2013 00:33:18 +0100
From:	Pavel Machek <pavel@....cz>
To:	Dave Chinner <david@...morbit.com>
Cc:	Josh Boyer <jwboyer@...oraproject.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Linux PM list <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>,
	linux-fsdevel@...r.kernel.org,
	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

Hi!

> > > > > I disagree - given the problem it is resolving leads to silent
> > > > > filesystem corruption, this patch should be considered somewhat of a
> > > > > priority to push...
> > > > 
> > > > Umm. Ok, I forgot what it does, really.
> > > 
> > > It ensures that the filesystem is in an quiescent state both in
> > > memory and on disk, and it cannot be modified in memory or on disk
> > > whilst the suspend image is being generated, or by log recovery
> > > after a resume before the suspended image has been restored.
> > 
> > If someone attempts to run log recovery before resume, that's a bug
> > and yes, it will corrupt filesystems. (Including ext3). Don't do that.
> 
> Freezing the filesystem prevents that accidental mount of the
> filesystem from being an issue. It fixes a bug that:

Can you elaborate on that?

If you do read-write mount of that filesystem, surely filesystem
metadata will differ from what the filesystem expects. You'll still
get data corruption AFAICT.

Read-only mount... maybe that will get slightly better -- there'll be
no journal to play back. But what happens to superblock information
such as "last mount time"? Mount counts?

> > Documentation/power/swsusp.txt:
> > 
> >  * BIG FAT WARNING
> >    *********************************************************
> >  *
> >  * If you touch anything on disk between suspend and resume...
> >  *                              ...kiss your data goodbye.
> 
> Makes this a whole lot less dangerous.

Do you claim that it is now safe to mount (rw) and access filesystem
between suspend and resume?

> > > > So... for few years now suspend corrupts data on XFS? And Fedora has
> > > > the fix but it is not in mainline? That does not sound right...
> > > 
> > > The issues freezing the filesystem before the suspend image is
> > > created affect every journalled filesystem linux supports, be
> > > it XFS, ext4, reiser, btrfs, etc.
> > 
> > Did not it have some problems with ext3?
> 
> Please read more carefully: "affect every journalled filesystem
> linux supports". So, ext3 is affected because it's a journalling
> filesystem.  I didn't list every journalled filesystem Linux
> supports - "etc" means there are more examples that aren't
> explicitly listed.

Sorry for being unclear. My memory says this patch caused some
regression with ext3. Is my memory wrong? Or was the regression fixed
in the meantime?

(Clearly, ext3 has the same issues with journal replays as btrfs. But
note that mounting filesystem between suspend and resume is not safe
even on simple filesystems such as ext2).

Thanks,
								Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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