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 dentry_create() to use atomic_open() for filesystems
that support it.  The previously const @path is passed up to atomic_open()
and may be modified depending on whether an existing entry was found or if
the atomic_open() returned an error and consumed the passed-in dentry.

Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/namei.c         | 46 +++++++++++++++++++++++++++++++++++++++-------
 fs/nfsd/nfs4proc.c |  8 +++++---
 include/linux/fs.h |  2 +-
 3 files changed, 45 insertions(+), 11 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 389f91a4d121..d70fd7362107 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -4286,6 +4286,9 @@ EXPORT_SYMBOL(start_creating_user_path);
  *
  * Caller must hold the parent directory's lock, and have prepared
  * a negative dentry, placed in @path->dentry, for the new file.
+ * If the file was looked up only or didn't need to be created,
+ * FMODE_OPENED will not be set, and @path will be updated with the
+ * new dentry.  The dentry may be negative.
  *
  * Caller sets @path->mnt to the vfsmount of the filesystem where
  * the new file is to be created. The parent directory and the
@@ -4294,21 +4297,50 @@ EXPORT_SYMBOL(start_creating_user_path);
  * 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 7f7e6bb23a90..7e39234e0649 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));
 
@@ -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 dd3b57cfadee..2d3fcb343993 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2883,7 +2883,7 @@ 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);
 const struct path *backing_file_user_path(const struct file *f);
 
-- 
2.50.1