Re: [PATCH] fs / ext3: Always unlock updates in ext3_freeze()
From: Rafael J. Wysocki
Date: Mon Aug 15 2011 - 18:06:23 EST
On Monday, August 15, 2011, Jan Kara wrote:
> 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...
> > > 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.
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