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

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Message-ID: <32c970d1f93a02207cb1746852e51bae4b2691c8.camel@themaw.net>
Date:   Thu, 30 Sep 2021 06:50:56 +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 Thu, 2021-09-30 at 06:28 +0800, Ian Kent wrote:
> 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.

But in case your ok to add it on my behalf it should be:
Fixes: c7e7c04274b1 ("kernfs: use VFS negative dentry caching")

> 
> 
> Ian