linux-ext4 - Re: [PATCH] fs / ext3: Always unlock updates in ext3

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Message-Id: <201108160007.50528.rjw@sisk.pl>
Date:	Tue, 16 Aug 2011 00:07:50 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH] fs / ext3: Always unlock updates in ext3_freeze()

Hi,

On Monday, August 15, 2011, Jan Kara wrote:
>   Hello,
> 
> On Mon 15-08-11 20:09:13, Rafael J. Wysocki wrote:
> > On Monday, August 15, 2011, Jan Kara wrote:
...
> > >   It's not so simple as this. Ext3 relies on the mutex (the one hidden in
> > > journal_lock_updates()) to make sure that new transaction cannot be started
> > > while the filesystem is frozen - that's essentially what makes the
> > > filesystem frozen. So if we want to get rid of the mutex we have to achieve
> > > blocking by something else - ext4 uses vfs_check_frozen() in
> > > ext4_journal_start().
> > 
> > I see.  Still, freeze_bdev() may be called by user space through a syscall,
> > as far as I can say, so it shouldn't leave the mutex locked.
>   Yes, I agree with you. That's an ugliness left over from a long time ago.
> I'll have a look at fixing this...

Thanks!

> > >   BTW,  filesystem freezing never really worked for mmaped writes under
> > > ext3 - ext3 would have to implement page_mkwrite() callback for that - so
> > > if you want to rely on it for suspending, this will be non-trivial.
> > 
> > At this point the purpose of freezing filesystems is basically to
> > prevent XFS from deadlocking with hibernation's memory preallocation.
> > For other filesystems it may or may not make a difference depending on
> > their implementation of freeze/unfreeze_super().
>   What's exactly the problem? Memory preallocation enters direct reclaim
> and that deadlocks in the filesystem?

Yes, that seems to be the case.

Thanks,
Rafael
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