Re: [PATCH v1 3/3] VFS/knfsd: Teach dentry_create() to use atomic_open()

[Mike Snitzer <snitzer@kernel.org>]

On Tue, Nov 18, 2025 at 11:33:59AM -0500, Benjamin Coddington wrote:
> While knfsd offers combined exclusive create and open results to clients,
> on some filesystems those results may not be atomic.  This behavior can be
> observed.  For example, an open O_CREAT with mode 0 will succeed in creating
> the file but unexpectedly return -EACCES from vfs_open().
> 
> Additionally reducing the number of remote RPC calls required for O_CREAT
> on network filesystem provides a performance benefit in the open path.
> 
> Teach knfsd's helper create_dentry() to use atomic_open() for filesystems
> that support it.
> 
> Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
> ---
>  fs/namei.c         | 43 ++++++++++++++++++++++++++++++++++++-------
>  fs/nfsd/nfs4proc.c |  8 +++++---
>  include/linux/fs.h |  2 +-
>  3 files changed, 42 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 9c0aad5bbff7..70ab74fb5e95 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -4208,21 +4208,50 @@ EXPORT_SYMBOL(user_path_create);
>   * On success, returns a "struct file *". Otherwise a ERR_PTR
>   * is returned.
>   */
> -struct file *dentry_create(const struct path *path, int flags, umode_t mode,
> +struct file *dentry_create(struct path *path, int flags, umode_t mode,
>  			   const struct cred *cred)
>  {
> +	struct dentry *dentry = path->dentry;
> +	struct dentry *dir = dentry->d_parent;
> +	struct inode *dir_inode = d_inode(dir);
> +	struct mnt_idmap *idmap;
>  	struct file *file;
> -	int error;
> +	int error, create_error;
>  
>  	file = alloc_empty_file(flags, cred);
>  	if (IS_ERR(file))
>  		return file;
>  
> -	error = vfs_create(mnt_idmap(path->mnt),
> -			   d_inode(path->dentry->d_parent),
> -			   path->dentry, mode, true);
> -	if (!error)
> -		error = vfs_open(path, file);
> +	idmap = mnt_idmap(path->mnt);
> +
> +	if (dir_inode->i_op->atomic_open) {
> +		path->dentry = dir;
> +		mode = vfs_prepare_mode(idmap, dir_inode, mode, S_IALLUGO, S_IFREG);
> +
> +		create_error = may_o_create(idmap, path, dentry, mode);
> +		if (create_error)
> +			flags &= ~O_CREAT;
> +
> +		dentry = atomic_open(path, dentry, file, flags, mode);
> +		error = PTR_ERR_OR_ZERO(dentry);
> +
> +		if (unlikely(create_error) && error == -ENOENT)
> +			error = create_error;
> +
> +		if (!error) {
> +			if (file->f_mode & FMODE_CREATED)
> +				fsnotify_create(dir->d_inode, dentry);
> +			if (file->f_mode & FMODE_OPENED)
> +				fsnotify_open(file);
> +		}
> +
> +		path->dentry = dentry;
> +
> +	} else {
> +		error = vfs_create(idmap, dir_inode, dentry, mode, true);
> +		if (!error)
> +			error = vfs_open(path, file);
> +	}
>  
>  	if (unlikely(error)) {
>  		fput(file);
> diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
> index 71b428efcbb5..7ff7e5855e58 100644
> --- a/fs/nfsd/nfs4proc.c
> +++ b/fs/nfsd/nfs4proc.c
> @@ -194,7 +194,7 @@ static inline bool nfsd4_create_is_exclusive(int createmode)
>  }
>  
>  static __be32
> -nfsd4_vfs_create(struct svc_fh *fhp, struct dentry *child,
> +nfsd4_vfs_create(struct svc_fh *fhp, struct dentry **child,
>  		 struct nfsd4_open *open)
>  {
>  	struct file *filp;
> @@ -214,9 +214,11 @@ nfsd4_vfs_create(struct svc_fh *fhp, struct dentry *child,
>  	}
>  
>  	path.mnt = fhp->fh_export->ex_path.mnt;
> -	path.dentry = child;
> +	path.dentry = *child;
>  	filp = dentry_create(&path, oflags, open->op_iattr.ia_mode,
>  			     current_cred());
> +	*child = path.dentry;
> +
>  	if (IS_ERR(filp))
>  		return nfserrno(PTR_ERR(filp));
>  

Given the potential for side-effect due to dentry_create() now using
atomic_open() if available, I think you'd do well to update the
comment block above dentry_create to make it clear that the caller
really should pass along the dentry (regardless of whether
dentry_create returns an ERR_PTR).

> @@ -353,7 +355,7 @@ nfsd4_create_file(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  	status = fh_fill_pre_attrs(fhp);
>  	if (status != nfs_ok)
>  		goto out;
> -	status = nfsd4_vfs_create(fhp, child, open);
> +	status = nfsd4_vfs_create(fhp, &child, open);
>  	if (status != nfs_ok)
>  		goto out;
>  	open->op_created = true;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 601d036a6c78..772b734477e5 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2878,7 +2878,7 @@ struct file *dentry_open(const struct path *path, int flags,
>  			 const struct cred *creds);
>  struct file *dentry_open_nonotify(const struct path *path, int flags,
>  				  const struct cred *cred);
> -struct file *dentry_create(const struct path *path, int flags, umode_t mode,
> +struct file *dentry_create(struct path *path, int flags, umode_t mode,
>  			   const struct cred *cred);
>  struct path *backing_file_user_path(const struct file *f);
>  
> -- 
> 2.50.1
>