Message-Id: <06dcc4b6-7457-4094-a1c6-586ce518020f@app.fastmail.com>
Date: Thu, 15 Jan 2026 16:37:27 -0500
From: "Chuck Lever" <cel@...nel.org>
To: "Dave Chinner" <david@...morbit.com>
Cc: "Amir Goldstein" <amir73il@...il.com>,
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Subject: Re: [PATCH 00/29] fs: require filesystems to explicitly opt-in to nfsd export
 support



On Thu, Jan 15, 2026, at 4:09 PM, Dave Chinner wrote:
> On Thu, Jan 15, 2026 at 02:37:09PM -0500, Chuck Lever wrote:
>> On 1/15/26 2:14 PM, Amir Goldstein wrote:
>> > On Thu, Jan 15, 2026 at 7:32 PM Chuck Lever <cel@...nel.org> wrote:
>> >>
>> >>
>> >>
>> >> On Thu, Jan 15, 2026, at 1:17 PM, Amir Goldstein wrote:
>> >>> On Thu, Jan 15, 2026 at 6:48 PM Jeff Layton <jlayton@...nel.org> wrote:
>> >>>>
>> >>>> In recent years, a number of filesystems that can't present stable
>> >>>> filehandles have grown struct export_operations. They've mostly done
>> >>>> this for local use-cases (enabling open_by_handle_at() and the like).
>> >>>> Unfortunately, having export_operations is generally sufficient to make
>> >>>> a filesystem be considered exportable via nfsd, but that requires that
>> >>>> the server present stable filehandles.
>> >>>
>> >>> Where does the term "stable file handles" come from? and what does it mean?
>> >>> Why not "persistent handles", which is described in NFS and SMB specs?
>> >>>
>> >>> Not to mention that EXPORT_OP_PERSISTENT_HANDLES was Acked
>> >>> by both Christoph and Christian:
>> >>>
>> >>> https://lore.kernel.org/linux-fsdevel/20260115-rundgang-leihgabe-12018e93c00c@brauner/
>> >>>
>> >>> Am I missing anything?
>> >>
>> >> PERSISTENT generally implies that the file handle is saved on
>> >> persistent storage. This is not true of tmpfs.
>> > 
>> > That's one way of interpreting "persistent".
>> > Another way is "continuing to exist or occur over a prolonged period."
>> > which works well for tmpfs that is mounted for a long time.
>> 
>> I think we can be a lot more precise about the guarantee: The file
>> handle does not change for the life of the inode it represents. It
>
> <pedantic mode engaged>
>
> File handles most definitely change over the life of a /physical/
> inode. Unlinking a file does not require ending the life of the
> physical object that provides the persistent data store for the
> file.
>
> e.g. XFS dynamically allocates physical inodes might in a life cycle
> that looks somewhat life this:
>
> 	allocate physical inode
> 	insert record into allocated inode index
> 	mark inode as free
>
> 	while (don't need to free physical inode) {
> 		...
> 		allocate inode for a new file
> 		update persistent inode metadata to generate new filehandle
> 		mark inode in use
> 		...
> 		unlink file
> 		mark inode free
> 	}
>
> 	remove inode from allocated inode index
> 	free physical inode
>
> i.e. a free inode is still an -allocated, indexed inode- in the
> filesystem, and until we physically remove it from the filesystem
> the inode life cycle has not ended.
>
> IOWs, the physical (persistent) inode lifetime can span the lifetime
> of -many- files. However, the filesystem guarantees that the handle
> generated for that inode is different for each file it represents
> over the whole inode life time.
>
> Hence I think that file handle stability/persistence needs to be
> defined in terms of -file lifetimes-, not the lifetimes of the
> filesystem objects implement the file's persistent data store.

Fair enough, "inode" is the wrong term to use here.


-- 
Chuck Lever

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