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Sender: Minchan Kim <minchan.kim@gmail.com>
Date:   Tue, 9 Feb 2021 23:12:04 -0800
From:   Minchan Kim <minchan@kernel.org>
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     John Hubbard <jhubbard@nvidia.com>,
        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
Subject: Re: [PATCH v2] mm: cma: support sysfs
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On Wed, Feb 10, 2021 at 07:43:37AM +0100, Greg KH wrote:
> On Tue, Feb 09, 2021 at 01:13:17PM -0800, Minchan Kim wrote:
> > On Tue, Feb 09, 2021 at 12:11:20PM -0800, John Hubbard wrote:
> > > 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,
> > 
> > Agreed. How about this for the warning part?
> > 
> > +
> > +/*
> > + * note: kobj_type should provide a release function to free dynamically
> > + * allocated object since kobject is responsible for controlling lifespan
> > + * of the object. However, cma_area is static object so technially, it
> > + * doesn't need release function. It's very exceptional case so pleaes
> > + * do not follow this model.
> > + */
> >  static struct kobj_type cma_ktype = {
> >         .sysfs_ops = &kobj_sysfs_ops,
> >         .default_groups = cma_groups
> > +       .release = NULL, /* do not follow. See above */
> >  };
> > 
> 
> No, please no.  Just do it the correct way, what is the objection to
> creating a few dynamic kobjects from the heap?  How many of these are
> you going to have that it will somehow be "wasteful"?
> 
> Please do it properly.

Oh, I misunderstood your word "don't provide a release function for the
kobject" so thought you agreed on John. If you didn't, we are stuck again:
IIUC, the objection from John was the cma_stat lifetime should be on parent
object, which is reasonable and make code simple.
Frankly speaking, I don't have strong opinion about either approach.
John?