linux-kernel - Re: [PATCH 05/14] tracefs: replace call_rcu by kfree_rcu for simple kmem_cache

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Message-ID: <05ec743a-c4e9-4c66-b2cd-4e89c858d7d4@suse.cz>
Date: Tue, 11 Jun 2024 10:42:28 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: Steven Rostedt <rostedt@...dmis.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Julia Lawall
 <Julia.Lawall@...ia.fr>, kernel-janitors@...r.kernel.org,
 Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 "workflows@...r.kernel.org" <workflows@...r.kernel.org>,
 Thorsten Leemhuis <linux@...mhuis.info>
Subject: Re: [PATCH 05/14] tracefs: replace call_rcu by kfree_rcu for simple
 kmem_cache_free callback

On 6/11/24 8:23 AM, Greg KH wrote:
> On Mon, Jun 10, 2024 at 11:40:54PM +0200, Vlastimil Babka wrote:
>> On 6/10/24 10:36 PM, Steven Rostedt wrote:
>> > On Mon, 10 Jun 2024 08:46:42 -0700
>> > "Paul E. McKenney" <paulmck@...nel.org> wrote:
>> > 
>> >> > > index 7c29f4afc23d..338c52168e61 100644
>> >> > > --- a/fs/tracefs/inode.c
>> >> > > +++ b/fs/tracefs/inode.c
>> >> > > @@ -53,14 +53,6 @@ static struct inode *tracefs_alloc_inode(struct super_block *sb)
>> >> > >  	return &ti->vfs_inode;
>> >> > >  }
>> >> > >  
>> >> > > -static void tracefs_free_inode_rcu(struct rcu_head *rcu)
>> >> > > -{
>> >> > > -	struct tracefs_inode *ti;
>> >> > > -
>> >> > > -	ti = container_of(rcu, struct tracefs_inode, rcu);
>> >> > > -	kmem_cache_free(tracefs_inode_cachep, ti);  
>> >> > 
>> >> > Does this work?
>> >> > 
>> >> > tracefs needs to be freed via the tracefs_inode_cachep. Does
>> >> > kfree_rcu() handle specific frees for objects that were not allocated
>> >> > via kmalloc()?  
>> >> 
>> >> A recent change to kfree() allows it to correctly handle memory allocated
>> >> via kmem_cache_alloc().  News to me as of a few weeks ago.  ;-)
>> > 
>> > If that's the case then:
>> > 
>> > Acked-by: Steven Rostedt (Google) <rostedt@...dmis.org>
>> > 
>> > Do we have a way to add a "Depends-on" tag so that anyone backporting this
>> > will know that it requires the change to whatever allowed that to happen?
>> 
>> Looks like people use that tag, although no grep hits in Documentation, so
>> Cc'ing workflows@ and Thorsten.
>> 
>> In this case it would be
>> 
>> Depends-on: c9929f0e344a ("mm/slob: remove CONFIG_SLOB")
> 
> Ick, no, use the documented way of handling this as described in the
> stable kernel rules file.

AFAICS that documented way is for a different situation? I assume you mean
this part:

* Specify any additional patch prerequisites for cherry picking::

    Cc: <stable@...r.kernel.org> # 3.3.x: a1f84a3: sched: Check for idle

But that would assume we actively want to backport this cleanup patch in the
first place. But as I understand Steven's intention, we want just to make
sure that if in the future this patch is backported (i.e. as a dependency of
something else) it won't be forgotten to also backport c9929f0e344a
("mm/slob: remove CONFIG_SLOB"). How to express that without actively
marking this patch for backport at the same time?

> thanks,
> 
> greg k-h