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

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Message-ID: <20250222041937.GM1977892@ZenIV>
Date: Sat, 22 Feb 2025 04:19:37 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: NeilBrown <neilb@...e.de>
Cc: Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Miklos Szeredi <miklos@...redi.hu>, Xiubo Li <xiubli@...hat.com>,
	Ilya Dryomov <idryomov@...il.com>,
	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>,
	Olga Kornievskaia <okorniev@...hat.com>,
	Dai Ngo <Dai.Ngo@...cle.com>, Tom Talpey <tom@...pey.com>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-cifs@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-um@...ts.infradead.org, ceph-devel@...r.kernel.org,
	netfs@...ts.linux.dev
Subject: Re: [PATCH 1/6] Change inode_operations.mkdir to return struct
 dentry *

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.

That's pretty much the same thing as d_drop()/d_rehash() window.

We'd be better off dropping that BUG_ON() in d_splice_alias() and teaching
__d_add() to handle the "it's a hashed negative" case.