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

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Message-Id: <201108240018.32189.rjw@sisk.pl>
Date:	Wed, 24 Aug 2011 00:18:32 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Dave Chinner <david@...morbit.com>
Cc:	Pavel Machek <pavel@....cz>, Jan Kara <jack@...e.cz>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs / ext3: Always unlock updates in ext3_freeze()

On Tuesday, August 23, 2011, Dave Chinner wrote:
> On Mon, Aug 22, 2011 at 03:00:45PM +0200, Pavel Machek wrote:
> > Hi!
> > 
> > > >   What's exactly the problem? Memory preallocation enters direct reclaim
> > > > and that deadlocks in the filesystem?
> > > 
> > > Well, that's one possible manifestation. The problem is that the
> > > current hibernate code still assumes that sys_sync() results in an
> > > idle filesystem that will not change after the call if nothing is
> > > dirty.
> > > 
> > > The result is that when the large memory allocation occurs for the
> > > hibernate image (after the sys_sync() call) then the shrink_slab()
> > > tends to be called. The XFS shrinkers are capable of dirtying inodes
> > > and the backing buffers of inodes that are in the reclaimable state.
> > > But those buffers cannot be flushed to disk because hibernate has
> > > already frozen the xfsbufd threads, so the shrinker doing inode
> > > reclaim hangs up on locks waiting for the buffers to be written.
> > > This either leads to deadlock or hibernate image allocation failure.
> > > 
> > > Far worse, IMO, is the case where is -doesn't- deadlock, because the
> > > filesystem state can still changing after the allocation has
> > > finished due to async metadata IO completions. That has the
> > > potential to cause filesystem corruption as after resume the on-disk
> > > state may not match what is written from memory to the hibernate
> > > image.
> > > 
> > > The problem really isn't XFS specific, nor is it new - the fact is
> > > that any filesystem that has registered a shrinker or can do async
> > > work in the background post-sync is vulnerable to this problem. It's
> > 
> > Should we avoid calling shrinkers while hibernating?
> 
> If you like getting random OOM problems when hibernating, then go
> for it.  Besides, shrinkers are used for more than just filesystems,
> so you might find you screw entire classes of users by doing this
> (eg everyone using intel graphics and 3D).
> 
> > Or put BUG_ON()s into filesystem shrinkers so that this can not
> > happen?
> 
> Definitely not. If your concern is filesystem shrinkers and you want
> a large hammer to hit the problem with then do your hibernate
> image allocation wih GFP_NOFS and the filesystem shrinkers will
> abort without doing anything.

I think we can do that, actually.

Thanks,
Rafael
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