linux-kernel - Re: [PATCH] kernfs: don't create a negative dentry if inactive node exists

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Message-ID: <d54d122a7267eddbdcdeb4cb4fad6630e9e0ffe3.camel@themaw.net>
Date:   Thu, 30 Sep 2021 06:28:08 +0800
From:   Ian Kent <raven@...maw.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Tejun Heo <tj@...nel.org>, Hou Tao <houtao1@...wei.com>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Carlos Maiolino <cmaiolino@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernfs: don't create a negative dentry if inactive node
 exists

On Wed, 2021-09-29 at 08:05 +0200, Greg Kroah-Hartman wrote:
> On Wed, Sep 29, 2021 at 11:04:34AM +0800, Ian Kent wrote:
> > In kernfs_iop_lookup() a negative dentry is created if there's no
> > kernfs
> > node associated with the dentry or the node is inactive.
> > 
> > But inactive kernfs nodes are meant to be invisible to the VFS and
> > creating a negative dentry for these can have unexpected side
> > effects
> > when the node transitions to an active state.
> > 
> > The point of creating negative dentries is to avoid the expensive
> > alloc/free cycle that occurs if there are frequent lookups for
> > kernfs
> > attributes that don't exist. So kernfs nodes that are not yet
> > active
> > should not result in a negative dentry being created so when they
> > transition to an active state VFS lookups can create an associated
> > dentry is a natural way.
> > 
> > Signed-off-by: Ian Kent <raven@...maw.net>
> > ---
> >  fs/kernfs/dir.c |    9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> Does this fix a specific commit and need a "Fixes:" tag?

Oh, of course yes, apologies, my bad.
I re-post it.


Ian