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Re: [PATCH 16/19] VFS: use GFP_NOFS rather than GFP_KERNEL in __d_alloc.

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 16/19] VFS: use GFP_NOFS rather than GFP_KERNEL in __d_alloc.
From: NeilBrown <neilb@xxxxxxx>
Date: Wed, 16 Apr 2014 16:49:41 +1000
Cc: linux-mm@xxxxxxxxx, linux-nfs@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140416062520.GG15995@dastard>
References: <20140416033623.10604.69237.stgit@xxxxxxxxxxxxxx> <20140416040337.10604.61837.stgit@xxxxxxxxxxxxxx> <20140416062520.GG15995@dastard>
On Wed, 16 Apr 2014 16:25:20 +1000 Dave Chinner <david@xxxxxxxxxxxxx> wrote:

> On Wed, Apr 16, 2014 at 02:03:37PM +1000, NeilBrown wrote:
> > __d_alloc can be called with i_mutex held, so it is safer to
> > use GFP_NOFS.
> > 
> > lockdep reports this can deadlock when loop-back NFS is in use,
> > as nfsd may be required to write out for reclaim, and nfsd certainly
> > takes i_mutex.
> 
> But not the same i_mutex as is currently held. To me, this seems
> like a false positive? If you are holding the i_mutex on an inode,
> then you have a reference to the inode and hence memory reclaim
> won't ever take the i_mutex on that inode.
> 
> FWIW, this sort of false positive was a long stabding problem for
> XFS - we managed to get rid of most of the false positives like this
> by ensuring that only the ilock is taken within memory reclaim and
> memory reclaim can't be entered while we hold the ilock.
> 
> You can't do that with the i_mutex, though....
> 
> Cheers,
> 
> Dave.

I'm not sure this is a false positive.
You can call __d_alloc when creating a file and so are holding i_mutex on the
directory.
nfsd might also want to access that directory.

If there was only 1 nfsd thread, it would need to get i_mutex and do it's
thing before replying to that request and so before it could handle the
COMMIT which __d_alloc is waiting for.

Obviously we would normally have multiple nfsd threads but if they were all
blocked on an i_mutex which itself was blocked on nfs_release_page which was
waiting for an nfsd thread to handling its COMMIT request, this could be a
real deadlock.

Thanks,
NeilBrown

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