Re: [PATCH 1/4] fs/notify: fdinfo can report unsupported file handles.

[Amir Goldstein]

On Mon, Dec 11, 2017 at 8:04 AM, NeilBrown <neilb@xxxxxxxx> wrote:
> If a filesystem does not set sb->s_export_op, then it
> does not support filehandles and export_fs_encode_fh()
> and exportfs_encode_inode_fh() should not be called.
> They will use export_encode_fh() is which is a default
> that uses inode number generation number, but in general
> they may not be stable.
>
> So change exportfs_encode_inode_fh() to return FILEID_INVALID
> if called on an unsupported Filesystem.  Currently only
> notify/fdinfo can do that.
>

I wish you would leave this check to the caller, maybe add a helper
exportfs_can_decode_fh() for callers to use.

Although there are no current uses for it in-tree, there is value in
being able to encode a unique file handle even when it cannot be
decoded back to an open file.

I am using this property in my fanotify super block watch patches,
where the object identifier on the event is an encoded file handle
of the object, which delegates tracking filesystem objects to
userspace and prevents fanotify from keeping elevated refcounts
on inodes and dentries.

There are quite a few userspace tools out there that are checking
that st_ino hasn't changed on a file between non atomic operations.
Those tools (or others) could benefit from a unique file handle if
we ever decide to provide a relaxed version of name_to_handle_at().

Cheers,
Amir.