Re: [PATCH 27/32] xfs: Add parent pointer ioctls

Darrick J. Wong
2024-04-16T18:52:09Z

On Tue, Apr 16, 2024 at 09:54:14AM -0700, Christoph Hellwig wrote:
> On Tue, Apr 16, 2024 at 09:50:56AM -0700, Darrick J. Wong wrote:
> > On Tue, Apr 16, 2024 at 06:47:16AM +0200, Christoph Hellwig wrote:
> > > On Mon, Apr 15, 2024 at 12:40:36PM -0700, Darrick J. Wong wrote:
> > > > True, libhandle is a very nice wrapper for the kernel ioctls. I wish
> > > > Linux projects did that more often. But suppose you're calling the
> > > > ioctls directly without libhandle and mess it up?
> > >
> > > The you get different inodes back. Not really any different from
> > > pointing your path name based code to the wrong fs or directory,
> > > is it?
> >
> > I suppose not. But why bother setting the fsid at all, then?
>
> I suspect that's a leftover from IRIX where the by handle operations
> weren't ioctls tied to a specific file system.

Oh, so on Irix a program could call the kernel with *only* the handle
and no fd? I wasn't aware of that, but most of my exposure to Irix was
wowwwing over the 3D File Explorer in _Jurassic Park_ and later an old
Indigo that someone donated to the high school. ;)

Ok, I'll drop the fsid checking code entirely.

> > > But as we never generate the file handles that encode the parent
> > > we already never connect files to their parent directory anyway.
> >
> > I pondered whether or not we should encode parent info in a regular
> > file's handle.
>
> We shouldn't. It's a really a NFS thing.
>
>
> > Would that result in an invalid handle if the file gets
> > moved to another directory?
>
> Yes.
>
> > That doesn't seem to fit with the behavior
> > that fds remain attached to the file even if it gets moved/deleted.
>
> Exactly.

<nod>

> >
> > > OTOH we should be able to optimize ->get_parent a bit with parent
> > > pointers, as we can find the name in the parent directory for
> > > a directory instead of doing linear scans in the parent directory.
> > > (for non-directory files we currenty don't fully connect anwyay)
> >
> > <nod> But does exportfs actually want parent info for a nondirectory?
> > There aren't any stubs or XXX/FIXME comments, and I've never heard any
> > calls (at least on fsdevel) for that functionality.
>
> It doesn't. It would avoid having disconnected dentries, but
> disconnected non-directory dentries aren't really a problem.

For directories, I think the dotdot lookup is much cheaper than scanning
the attrs to find the first nongarbage XFS_ATTR_PARENT entry.

--D