On Tue, Feb 09, 2021 at 07:56:30AM +0100, Greg KH wrote:

On Mon, Feb 08, 2021 at 10:34:51PM -0800, John Hubbard wrote:

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On 2/8/21 10:27 PM, John Hubbard wrote:

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On 2/8/21 10:13 PM, Greg KH wrote:

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On Mon, Feb 08, 2021 at 05:57:17PM -0800, John Hubbard wrote:

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On 2/8/21 3:36 PM, Minchan Kim wrote:
...

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        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?

Sorry, I think I get it now: this is in order to allow a separate lifetime
for the .stat member. I was sort of implicitly assuming that the "right"
way to do it is just have the whole object use one lifetime management,
but as you say, there is no kobject being added to the parent.

I still feel odd about the allocation and freeing of something that seems
to be logically the same lifetime (other than perhaps a few, briefly pending
sysfs reads, at the end of life). So I'd still think that the kobject should
be added to the parent...

sruct cma_stat {
	spinlock_t lock;
	unsigned long pages_attemtp;
	unsigned long pages_fail;
};

struct cma {
	..
	..
	struct kobject kobj;
	struct cma_stat stat;
};

I guess this is what Johan suggested. I agree with it.

That's fine if you want to add it to the parent.  If so, then the
kobject controls the lifetime of the structure, nothing else can.

The problem was parent object(i.e., struct cma cma_areas) is
static arrary so kobj->release function will be NULL or just
dummy. Is it okay? I thought it was one of the what you wanted
to avoid it.

Either is fine with me, what is "forbidden" is having a kobject and
somehow thinking that it does not control the lifetime of the structure.

Since parent object is static arrary, there is no need to control the
lifetime so I am curious if parent object approach is okay from kobject
handling point of view.

If it's no problem, I am happy to change it.