linux-kernel - Re: [PATCH v4] mm: cma: support sysfs

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Date:   Fri, 19 Mar 2021 22:24:03 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Minchan Kim <minchan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, joaodias@...gle.com,
        willy@...radead.org, david@...hat.com, surenb@...gle.com,
        John Hubbard <jhubbard@...dia.com>,
        Nicolas Chauvet <kwizart@...il.com>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH v4] mm: cma: support sysfs

19.03.2021 22:03, Minchan Kim пишет:
> On Fri, Mar 19, 2021 at 09:48:11PM +0300, Dmitry Osipenko wrote:
>> 19.03.2021 21:21, Minchan Kim пишет:
>>> On Fri, Mar 19, 2021 at 08:56:06PM +0300, Dmitry Osipenko wrote:
>>>> 19.03.2021 19:30, Minchan Kim пишет:
>>>>> +static void cma_kobj_release(struct kobject *kobj)
>>>>> +{
>>>>> +	struct cma_kobject *cma_kobj = container_of(kobj, struct cma_kobject, kobj);
>>>>> +
>>>>> +	kfree(cma_kobj);
>>>>> +}
>>>>
>>>> Oh, wait.. I think this kfree wrong since cma_kobj belongs to the array.
>>>
>>> Oh, good spot. Let me use kzalloc.
>>>
>>
>> Thinking a bit more about this.. it looks like actually it should be
>> better to get back to the older variant of cma_stat, but allocate at the
>> time of CMA initialization, rather than at the time of sysfs
>> initialization. Then the cma_stat will be decoupled from the cma struct
> 
> IIRC, the problem was slab was not initiaized at CMA init point.
> That's why I liked your suggestion.

Alright, if CMA init time is a problem, then the recent variant should
be okay.

>> and cma_stat will be a self-contained object.
> 
> Yeah, self-contained is better but it's already weird to
> have differnt lifetime for one object since CMA object
> never die, technically.
> 

Indeed.

I found the Greg's original argument and not sure that it's really
worthwhile to worry about the copycats since this is not a driver's code..

Maybe we could just add a clarifying comment for the kobj, telling why
it's okay for CMA. Greg, doesn't it sound like a good compromise to you?