Re: [PATCH 08/11] VFS: allow d_splice_alias() and d_add() to work on hashed dentries.

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

On Tue, Aug 12, 2025 at 12:25:11PM +1000, NeilBrown wrote:
> Proposed locking changes will require a dentry to remain hashed during
> all directory operations which are currently protected by i_rwsem, or
> for there to be a controlled transition from one hashed dentry to
> another which maintains the lock - which will then be on the dentry.
> 
> The current practice of dropping (unhashing) a dentry before calling
> d_splice_alias() and d_add() defeats this need.
> 
> This patch changes d_splice_alias() and d_add() to accept a hashed
> dentry and to only drop it when necessary immediately before an
> alternate dentry is hashed.  These functions will, in a subsequent patch,
> transfer the dentry locking across so that the name remains locked in
> the directory.

The problem I have with that is the loss of information.  That is to
say, "is it hashed here" is hard to deduce from code.  I would rather
add d_splice_alias_hashed() and d_add_hashed(), and then see what's
going on in specific callers.  

And yes, it requires analysis of places where we have d_drop()+d_add() -
better have it done upfront than repeat it on every code audit *for*
*each* *single* *call* *of* d_add(), including the ones that are currently
obviously meant to be called for unhashed dentries.

I realize that it's no fun at all - in particular, I'd never been able
to get ceph folks to explain what is and what is not possible there.

I would really hate to have that expand by order of magnitude - in
effect, you make *all* calls of d_splice_alias() and d_add() similar
mysteries.