Re: [PATCH 1/6] Change inode_operations.mkdir to return struct dentry *

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, Feb 24, 2025 at 12:34:06PM +1100, NeilBrown wrote:
> On Sat, 22 Feb 2025, Al Viro wrote:
> > On Fri, Feb 21, 2025 at 10:36:30AM +1100, NeilBrown wrote:
> > 
> > > +In general, filesystems which use d_instantiate_new() to install the new
> > > +inode can safely return NULL.  Filesystems which may not have an I_NEW inode
> > > +should use d_drop();d_splice_alias() and return the result of the latter.
> > 
> > IMO that's a bad pattern, _especially_ if you want to go for "in-update"
> > kind of stuff later.
> 
> Agreed.  I have a draft patch to change d_splice_alias() and
> d_exact_alias() to work on hashed dentrys.  I thought it should go after
> these mkdir patches rather than before.

Could you give a braindump on the things d_exact_alias() is needed for?
It's a recurring headache when doing ->d_name/->d_parent audits; see e.g.
https://lore.kernel.org/all/20241213080023.GI3387508@ZenIV/ for related
mini-rant from the latest iteration.

Proof of correctness is bloody awful; it feels like the primitive itself
is wrong, but I'd never been able to write anything concise regarding
the things we really want there ;-/