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Subject: Re: [PATCH v2] mm: cma: support sysfs
To:     Greg KH <gregkh@linuxfoundation.org>
CC:     Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, <surenb@google.com>,
        <joaodias@google.com>, <willy@infradead.org>
References: <20210208180142.2765456-1-minchan@kernel.org>
 <e01c111b-fb20-0586-c7a9-dd6d922c0e57@nvidia.com>
 <YCHLAdabGmm7kqSH@google.com>
 <43cd6fc4-5bc5-50ec-0252-ffe09afd68ea@nvidia.com>
 <YCIoHBGELFWAyfMi@kroah.com>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <7cc229f4-609c-71dd-9361-063ef1bf7c73@nvidia.com>
Date:   Mon, 8 Feb 2021 22:27:24 -0800
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On 2/8/21 10:13 PM, Greg KH wrote:
> On Mon, Feb 08, 2021 at 05:57:17PM -0800, John Hubbard wrote:
>> On 2/8/21 3:36 PM, Minchan Kim wrote:
>> ...
>>>>>     	char name[CMA_MAX_NAME];
>>>>> +#ifdef CONFIG_CMA_SYSFS
>>>>> +	struct cma_stat	*stat;
>>>>
>>>> This should not be a pointer. By making it a pointer, you've added a bunch of pointless
>>>> extra code to the implementation.
>>>
>>> Originally, I went with the object lifetime with struct cma as you
>>> suggested to make code simple. However, Greg KH wanted to have
>>> release for kobj_type since it is consistent with other kboject
>>> handling.
>>
>> Are you talking about the kobj in your new struct cma_stat? That seems
>> like circular logic if so. I'm guessing Greg just wanted kobj methods
>> to be used *if* you are dealing with kobjects. That's a narrower point.
>>
>> I can't imagine that he would have insisted on having additional
>> allocations just so that kobj freeing methods could be used. :)
> 
> Um, yes, I was :)
> 
> You can not add a kobject to a structure and then somehow think you can
> just ignore the reference counting issues involved.  If a kobject is
> part of a structure then the kobject is responsible for controling the
> lifespan of the memory, nothing else can be.
> 
> So by making the kobject dynamic, you properly handle that memory
> lifespan of the object, instead of having to worry about the lifespan of
> the larger object (which the original patch was not doing.)
> 
> Does that make sense?
> 
That part makes sense, yes, thanks. The part that I'm trying to straighten
out is, why was kobject even added to the struct cma_stat in the first
place? Why not just leave .stat as a static member variable, without
a kobject in it, and done?

thanks,
-- 
John Hubbard
NVIDIA