Re: [PATCH] nfsd: Prevent truncation of an unlinked inode from blocking access to its directory

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, May 14, 2021 at 11:58:29AM +0800, Nick Huang wrote:
> From: Yu Hsiang Huang <nickhuang@synology.com>
> 
> Truncation of an unlinked inode may take a long time for I/O waiting, and
> it doesn't have to prevent access to the directory. Thus, let truncation
> occur outside the directory's mutex, just like do_unlinkat() does.

Makes sense to me, thanks.  I'll queue it up for 5.14.

Just out of curiosity: was this found just by inspection, or were you
hitting this in a real workload?  I'd be interested in what it took to
reproduce if so.

--b.

> 
> Signed-off-by: Yu Hsiang Huang <nickhuang@synology.com>
> Signed-off-by: Bing Jing Chang <bingjingc@synology.com>
> Signed-off-by: Robbie Ko <robbieko@synology.com>
> ---
>  fs/nfsd/vfs.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 15adf1f6ab21..39948f130712 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -1859,6 +1859,7 @@ nfsd_unlink(struct svc_rqst *rqstp, struct svc_fh *fhp, int type,
>  {
>  	struct dentry	*dentry, *rdentry;
>  	struct inode	*dirp;
> +	struct inode	*rinode;
>  	__be32		err;
>  	int		host_err;
>  
> @@ -1887,6 +1888,8 @@ nfsd_unlink(struct svc_rqst *rqstp, struct svc_fh *fhp, int type,
>  		host_err = -ENOENT;
>  		goto out_drop_write;
>  	}
> +	rinode = d_inode(rdentry);
> +	ihold(rinode);
>  
>  	if (!type)
>  		type = d_inode(rdentry)->i_mode & S_IFMT;
> @@ -1902,6 +1905,8 @@ nfsd_unlink(struct svc_rqst *rqstp, struct svc_fh *fhp, int type,
>  	if (!host_err)
>  		host_err = commit_metadata(fhp);
>  	dput(rdentry);
> +	fh_unlock(fhp);
> +	iput(rinode);    /* truncate the inode here */
>  
>  out_drop_write:
>  	fh_drop_write(fhp);
> -- 
> 2.25.1