linux-kernel - Re: [PATCH v1 0/3] Allow knfsd to use atomic

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Message-ID: <050d60a8-7689-46f3-a303-28e01944b386@oracle.com>
Date: Tue, 18 Nov 2025 11:58:28 -0500
From: Chuck Lever <chuck.lever@...cle.com>
To: Benjamin Coddington <bcodding@...merspace.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>, NeilBrown <neil@...wn.name>,
        Olga Kornievskaia <okorniev@...hat.com>, Dai Ngo <Dai.Ngo@...cle.com>,
        Tom Talpey <tom@...pey.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nfs@...r.kernel.org, Trond Myklebust <trondmy@...nel.org>,
        Mike Snitzer <snitzer@...nel.org>
Subject: Re: [PATCH v1 0/3] Allow knfsd to use atomic_open()

On 11/18/25 11:33 AM, Benjamin Coddington wrote:
> We have workloads that will benefit from allowing knfsd to use atomic_open()
> in the open/create path.  There are two benefits; the first is the original
> matter of correctness: when knfsd must perform both vfs_create() and
> vfs_open() in series there can be races or error results that cause the
> caller to receive unexpected results.

Commit fb70bf124b05 ("NFSD: Instantiate a struct file when creating a
regular NFSv4 file") was supposed to address this. If there are still
issues, then a Fixes: tag and some explanation of where there are gaps
would be welcome in the commit message or cover letter. We might need
to identify LTS backport requirements, in that case.


> The second benefit is that for some
> network filesystems, we can reduce the number of remote round-trip
> operations by using a single atomic_open() path which provides a performance
> benefit. 
> 
> I've implemented this with the simplest possible change - by modifying
> dentry_create() which has a single user: knfsd.  The changes cause us to
> insert ourselves part-way into the previously closed/static atomic_open()
> path, so I expect VFS folks to have some good ideas about potentially
> superior approaches.
> 
> Thanks for any comment and critique.
> 
> Benjamin Coddington (3):
>   VFS: move dentry_create() from fs/open.c to fs/namei.c
>   VFS: Prepare atomic_open() for dentry_create()
>   VFS/knfsd: Teach dentry_create() to use atomic_open()
> 
>  fs/namei.c         | 84 ++++++++++++++++++++++++++++++++++++++++++----
>  fs/nfsd/nfs4proc.c |  8 +++--
>  fs/open.c          | 41 ----------------------
>  include/linux/fs.h |  2 +-
>  4 files changed, 83 insertions(+), 52 deletions(-)
> 


-- 
Chuck Lever