linux-kernel - Re: [PATCH 08/11] VFS: allow d_splice_alias() and d

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Message-ID: <20250813050717.GD222315@ZenIV>
Date: Wed, 13 Aug 2025 06:07:17 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: NeilBrown <neil@...wn.name>
Cc: Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	David Howells <dhowells@...hat.com>,
	Marc Dionne <marc.dionne@...istor.com>,
	Xiubo Li <xiubli@...hat.com>, Ilya Dryomov <idryomov@...il.com>,
	Tyler Hicks <code@...icks.com>, Miklos Szeredi <miklos@...redi.hu>,
	Richard Weinberger <richard@....at>,
	Anton Ivanov <anton.ivanov@...bridgegreys.com>,
	Johannes Berg <johannes@...solutions.net>,
	Trond Myklebust <trondmy@...nel.org>,
	Anna Schumaker <anna@...nel.org>,
	Chuck Lever <chuck.lever@...cle.com>,
	Jeff Layton <jlayton@...nel.org>,
	Amir Goldstein <amir73il@...il.com>,
	Steve French <sfrench@...ba.org>,
	Namjae Jeon <linkinjeon@...nel.org>,
	Carlos Maiolino <cem@...nel.org>, linux-fsdevel@...r.kernel.org,
	linux-afs@...ts.infradead.org, netfs@...ts.linux.dev,
	ceph-devel@...r.kernel.org, ecryptfs@...r.kernel.org,
	linux-um@...ts.infradead.org, linux-nfs@...r.kernel.org,
	linux-unionfs@...r.kernel.org, linux-cifs@...r.kernel.org,
	linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/11] VFS: allow d_splice_alias() and d_add() to work on
 hashed dentries.

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.