linux-kernel - Re: [PATCH v2] mm: cma: support sysfs

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Date:   Wed, 10 Feb 2021 07:43:37 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, surenb@...gle.com,
        joaodias@...gle.com, willy@...radead.org
Subject: Re: [PATCH v2] mm: cma: support sysfs

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.

thanks,

greg k-h