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

From: Greg KH <gregkh@linuxfoundation.org>
To: John Hubbard <jhubbard@nvidia.com>
Cc: Minchan Kim <minchan@kernel.org>,
	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
Date: Wed, 10 Feb 2021 08:26:28 +0100	[thread overview]
Message-ID: <YCOKpM9lufhD/myy@kroah.com> (raw)
In-Reply-To: <e7ea55b9-a5f6-0daf-843b-e25d8c70e980@nvidia.com>

On Tue, Feb 09, 2021 at 11:16:07PM -0800, John Hubbard wrote:
> On 2/9/21 11:12 PM, Minchan Kim wrote:
> ...
> > > > 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?
> > 
> 
> We should do it as Greg requests, now that it's quite clear that he's insisting
> on this. Not a big deal.
> 
> I just am not especially happy about the inability to do natural, efficient
> things here, such as use a statically allocated set of things with sysfs. And
> I remain convinced that the above is not "improper"; it's a reasonable
> step, given the limitations of the current sysfs design. I just wanted to say
> that out loud, as my proposal sinks to the bottom of the trench here. haha :)

What is "odd" is that you are creating an object in the kernel that you
_never_ free.  That's not normal at all in the kernel, and so, your wish
to have a kobject that you never free represent this object also is not
normal :)

thanks,

greg k-h