Re: [PATCH 2/2] fs/dcache: allow __d_obtain_alias() to return unhashed dentries

[Nick Piggin <npiggin@kernel.dk>]

On Fri, Dec 17, 2010 at 01:00:23PM -0500, J. Bruce Fields wrote:
> From: J. Bruce Fields <bfields@redhat.com>
> 
> Without this patch
> 
>        client$ mount -tnfs4 server:/export/ /mnt/
>        client$ tail -f /mnt/FOO
>        ...
>        server$ df -i /export
>        server$ rm /export/FOO
>        (^C the tail -f)
>        server$ df -i /export
>        server$ echo 2 >/proc/sys/vm/drop_caches
>        server$ df -i /export
> 
> the df's will show that the inode is not freed on the filesystem until
> the last step, when it could have been freed after killing the client's
> tail -f.  On-disk data won't be deallocated either, leading to possible
> spurious ENOSPC.
> 
> This occurs because when the client does the close, it arrives in a
> compound with a putfh and a close, processed like:
> 
>        - putfh: look up the filehandle.  The only alias found for the
>          inode will be DCACHE_UNHASHED alias referenced by the filp
>          associated with the nfsd open.  d_obtain_alias() doesn't like
>          this, so it creates a new DCACHE_DISCONECTED dentry and
>          returns that instead.
> 
> Nick Piggin suggested fixing this by allowing d_obtain_alias to return
> the unhashed dentry that is referenced by the filp, instead of making it
> create a new dentry.
> 
> Leave __d_find_alias() alone to avoid changing behavior of other
> callers.
> 
> Also nfsd doesn't need all the checks of __d_find_alias(); any dentry,
> hashed or unhashed, disconnected or not, should work.
> 
> Signed-off-by: J. Bruce Fields <bfields@redhat.com>
> ---
>  fs/dcache.c |   26 ++++++++++++++++++++++++--
>  1 files changed, 24 insertions(+), 2 deletions(-)
> 
> On Tue, Dec 14, 2010 at 05:01:02PM -0500, J. Bruce Fields wrote:
> > On Mon, Dec 13, 2010 at 04:19:44PM +1100, Nick Piggin wrote:
> > > Not sure where Al's hiding...
> > > 
> > > But I would like to update the comments, and perhaps even a new
> > > add a new function here (or new flag to __d_find_alias).
> > > 
> > > AFAIKS, the callers are OK, however I suppose d_splice_alias and
> > > d_materialise_unique should not have unlinked inodes at this point,
> > > so at least a BUG_ON for them might be a good idea?
> > 
> > That does sound safer.  I'm pretty confused by the various
> > __di_splice_alias callers.  I'll go search through them and see if I can
> > understand better....
> 
> I think a new __d_find_alias flag would just make it more confusing.
> And it looks like all d_obtain_alias needs is something much simpler.
> This works for me.

Well I don't really know if this is clearer. By convention,
__ prefix version should be same as non prefix version but just
take fewer locks. Why do you have the new d_find_any_alias()?



> 
> --b.
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 5ed93cd..d5f9da4 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1135,6 +1135,28 @@ static inline struct hlist_head *d_hash(struct dentry *parent,
>  	return dentry_hashtable + (hash & D_HASHMASK);
>  }
>  
> +static struct dentry * __d_find_any_alias(struct inode *inode)
> +{
> +	struct dentry *alias;
> +
> +	if (list_empty(&inode->i_dentry))
> +		return NULL;
> +	alias = list_first_entry(&inode->i_dentry, struct dentry, d_alias);
> +	__dget_locked(alias);
> +	return alias;
> +}
> +
> +static struct dentry * d_find_any_alias(struct inode *inode)
> +{
> +	struct dentry *de;
> +
> +	spin_lock(&dcache_lock);
> +	de = __d_find_alias(inode, 0);
> +	spin_unlock(&dcache_lock);
> +	return de;
> +}
> +
> +
>  /**
>   * d_obtain_alias - find or allocate a dentry for a given inode
>   * @inode: inode to allocate the dentry for
> @@ -1164,7 +1186,7 @@ struct dentry *d_obtain_alias(struct inode *inode)
>  	if (IS_ERR(inode))
>  		return ERR_CAST(inode);
>  
> -	res = d_find_alias(inode);
> +	res = d_find_any_alias(inode);
>  	if (res)
>  		goto out_iput;
>  
> @@ -1176,7 +1198,7 @@ struct dentry *d_obtain_alias(struct inode *inode)
>  	tmp->d_parent = tmp; /* make sure dput doesn't croak */
>  
>  	spin_lock(&dcache_lock);
> -	res = __d_find_alias(inode, 0);
> +	res = __d_find_any_alias(inode);
>  	if (res) {
>  		spin_unlock(&dcache_lock);
>  		dput(tmp);
> -- 
> 1.7.1