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

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Message-ID: <a578b2e33aa145be3f0d63a23bc7dfe2bd56d0ab.camel@kernel.org>
Date: Tue, 18 Nov 2025 12:45:13 -0500
From: Trond Myklebust <trondmy@...nel.org>
To: Chuck Lever <chuck.lever@...cle.com>, 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, Mike Snitzer <snitzer@...nel.org>
Subject: Re: [PATCH v1 0/3] Allow knfsd to use atomic_open()

On Tue, 2025-11-18 at 11:58 -0500, Chuck Lever wrote:
> 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.
> 

That patch only fixes the case where you're creating a local file and
then exporting it over NFSv4.

The case where we see a permissions problem is when creating a file
over NFSv4, and then exporting it over NFSv3.
i.e. it is the re-exporting over NFSv3 case.

Note that independently of the permissions issues, atomic_open also
solves races in open(O_CREAT|O_TRUNC). The NFS client now uses it for
both NFSv4 and NFSv3 for that reason.
See commit 7c6c5249f061 "NFS: add atomic_open for NFSv3 to handle
O_TRUNC correctly."

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trondmy@...nel.org, trond.myklebust@...merspace.com