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

[John Hubbard]

On 2/9/21 9:49 AM, Greg KH wrote:
> > > 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.
>
> No, that is not ok.
>
> > > 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.
>
> So the array is _NEVER_ freed?  If not, fine, don't provide a release
> function for the kobject, but ick, just make a dynamic kobject I don't
> see the problem for something so tiny and not very many...

Yeah, I wasn't trying to generate so much discussion, I initially thought it
would be a minor comment: "just use an embedded struct and avoid some extra
code", at first.

> I worry that any static kobject might be copied/pasted as someone might
> think this is an ok thing to do.  And it's not an ok thing to do.

Overall, then, we're seeing that there is a small design hole: in order
to use sysfs most naturally, you either much provide a dynamically allocated
item for it, or you must use the static kobject, and the second one sets a
bad example.

I think we should just use a static kobject, with a cautionary comment to
would-be copy-pasters, that explains the design constraints above. That way,
we still get a nice, less-code implementation, a safe design, and it only
really costs us a single carefully written comment.

thanks,
--
John Hubbard
NVIDIA