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

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Date: Tue, 11 Jun 2024 11:05:25 +0200
From: Thorsten Leemhuis <linux@...mhuis.info>
To: Vlastimil Babka <vbabka@...e.cz>, 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>
Subject: Re: [PATCH 05/14] tracefs: replace call_rcu by kfree_rcu for simple
 kmem_cache_free callback

On 11.06.24 10:42, Vlastimil Babka wrote:
> 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?

Hah, waiting a bit spared me the time to write a similar reply. :-D
Writing one now anyway to broaden the scope:

I recently noticed we have the same problem when it comes to the
"delayed backporting" aspect, e.g. this part:

"""
* Delay pick up of patches::

    Cc: <stable@...r.kernel.org> # after -rc3
"""

I'll bring this up in a maintainers summit proposal I'm currently
preparing. But I have no idea how to solve this in an elegant way.
"Cc: <stable+INeitherKnowNorCare@...nel.org> # after -rc3" could work,
but well, as indicated, that's kinda ugly.

Ciao, Thorsten