Re: [PATCH v1 0/3] Allow knfsd to use atomic_open()

[Benjamin Coddington]

On 18 Nov 2025, at 20:23, NeilBrown wrote:

> On Wed, 19 Nov 2025, 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.  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.
>
> I think using atomic_open is important - thanks for doing this.
>
> I think there is another race this fixes.
> If the client ends and unchecked v4 OPEN request, nfsd does a lookup and
> finds the name doesn't exist, it will then (currently) use vfs_create()
> requesting an exclusive create.  If this races with a create happening
> from another client, this could result in -EEXIST which is not what the
> client would expect.  Using atomic_open would fix this.
>
> However I cannot see that you ever pass O_EXCL to atomic_open (or did I
> miss something?).  So I don't think the code is quite right yet.  O_EXCL
> should be passed is an exclusive or checked create was requested.

Ah, it's true.  I did not validate knfsd's behaviors, only its interface with
VFS.  IIUC knfsd gets around needing to pass O_EXCL by holding the directory
inode lock over the create, and since it doesn't need to do lookup because
it already has a filehandle, I think O_EXCL is moot.

> With a VFS hat on, I would rather there were more shared code between
> dentry_create() and lookup_open().  I don't know exactly what this would
> look like, and I wouldn't want that desire to hold up this patch, but it
> might be worth thinking about to see if there are any easy similarities
> to exploit.

I agree, that would be nice.  It would definitely be a bigger touch, and I
was going for the minimal change here.

Thanks for looking at this Neil.

Ben