linux-kernel - Re: NFS lockdep lock misordering mmap_sem<->i_mutex

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Date:	Tue, 15 Dec 2009 18:54:37 -0500
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: NFS lockdep lock misordering mmap_sem<->i_mutex_key with
 2.6.32-git1

On Wed, 2009-12-16 at 00:38 +0100, Andi Kleen wrote: 
> I suppose an easy workaround would be to not revalidate in mmap,
> because open should have already done that?
> 
> Very lightly tested RFC patch attached.
> 
> -Andi
> 
> ---
> 
> NFS: don't revalidate in mmap
> 
> nfs_revalidate_mapping takes i_mutex, but mmap already has mmap_sem
> hold and taking i_mutex inside mmap_sem is not allowed by the VFS.
> 
> So don't revalidate on mmap time and trust it has been already done.
> 
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> 
> ---
>  fs/nfs/file.c |    7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
> 
> Index: linux-2.6.32-ak/fs/nfs/file.c
> ===================================================================
> --- linux-2.6.32-ak.orig/fs/nfs/file.c
> +++ linux-2.6.32-ak/fs/nfs/file.c
> @@ -297,14 +297,9 @@ nfs_file_mmap(struct file * file, struct
>  	dprintk("NFS: mmap(%s/%s)\n",
>  		dentry->d_parent->d_name.name, dentry->d_name.name);
>  
> -	/* Note: generic_file_mmap() returns ENOSYS on nommu systems
> -	 *       so we call that before revalidating the mapping
> -	 */
>  	status = generic_file_mmap(file, vma);
> -	if (!status) {
> +	if (!status)
>  		vma->vm_ops = &nfs_file_vm_ops;
> -		status = nfs_revalidate_mapping(inode, file->f_mapping);
> -	}
>  	return status;
>  }
>  

If you want to work around the problem rather than going for something
like Peter's split up of the mmap() callback, then I'd suggest changing
to using nfs_revalidate_mapping_nolock() instead. The fact that we are
seeing these lock misordering warnings is proof that the call to
nfs_revalidate_mapping() is not always a no-op.

By not taking the i_mutex your call to invalidate_inode_pages2() can
potentially end up racing with another process that is writing to the
file, but that should be a rare occurrence. The effect will be that the
two processes can end up fighting to alternatively dirty and then clean
the pages...

Cheers
  Trond

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