linux-kernel - Re: [PATCH 04/11] VFS: introduce dentry_lookup

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Message-ID: <20250813042202.GA222315@ZenIV>
Date: Wed, 13 Aug 2025 05:22:02 +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 04/11] VFS: introduce dentry_lookup_continue()

On Tue, Aug 12, 2025 at 12:25:07PM +1000, NeilBrown wrote:
> A few callers operate on a dentry which they already have - unlike the
> normal case where a lookup proceeds an operation.
> 
> For these callers dentry_lookup_continue() is provided where other
> callers would use dentry_lookup().  The call will fail if, after the
> lock was gained, the child is no longer a child of the given parent.
> 
> There are a couple of callers that want to lock a dentry in whatever
> its current parent is.  For these a NULL parent can be passed, in which
> case ->d_parent is used.  In this case the call cannot fail.
> 
> The idea behind the name is that the actual lookup occurred some time
> ago, and now we are continuing with an operation on the dentry.
> 
> When the operation completes done_dentry_lookup() must be called.  An
> extra reference is taken when the dentry_lookup_continue() call succeeds
> and will be dropped by done_dentry_lookup().
> 
> This will be used in smb/server, ecryptfs, and overlayfs, each of which
> have their own lock_parent() or parent_lock() or similar; and a few
> other places which lock the parent but don't check if the parent is
> still correct (often because rename isn't supported so parent cannot be
> incorrect).

I would really like the see the conversion of these callers.  You are
asking for a buy-in for a primitive with specific semantics; that's
hard to review without seeing how it will be used.