Re: [PATCH v2] mm: cma: support sysfs

[Minchan Kim <minchan@kernel.org>]

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