linux-kernel - Re: [lkp] [gpio] 3c702e9987: kmsg.user_verbs:couldn't_register_device

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Date:	Sun, 14 Feb 2016 22:21:50 -0600
From:	Michael Welling <mwelling@...e.org>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	Roland Dreier <rolandd@...co.com>,
	kernel test robot <ying.huang@...ux.intel.com>, lkp@...org,
	LKML <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Markus Pargmann <mpa@...gutronix.de>,
	Johan Hovold <johan@...nel.org>
Subject: Re: [lkp] [gpio] 3c702e9987:
 kmsg.user_verbs:couldn't_register_device_number

On Sun, Feb 14, 2016 at 03:49:54PM -0800, Greg KH wrote:
> On Sun, Feb 14, 2016 at 01:51:20PM -0600, Michael Welling wrote:
> > On Sun, Feb 14, 2016 at 11:05:15AM -0800, Greg KH wrote:
> > > On Sun, Feb 14, 2016 at 06:56:20PM +0100, Linus Walleij wrote:
> > > > On Sun, Feb 14, 2016 at 6:49 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
> > > > > On Sun, Feb 14, 2016 at 06:42:11PM +0100, Linus Walleij wrote:
> > > > >> Greg, heads-up on this... you'd know if this happened
> > > > >> before.
> > > > >>
> > > > >> On Sun, Feb 14, 2016 at 9:06 AM, Michael Welling <mwelling@...e.org> wrote:
> > > > >> > On Sun, Feb 14, 2016 at 02:59:06PM +0800, kernel test robot wrote:
> > > > >> >> FYI, we noticed the below changes on
> > > > >> >>
> > > > >> >> https://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-gpio.git chardev
> > > > >> >> commit 3c702e9987e261042a07e43460a8148be254412e ("gpio: add a userspace chardev ABI for GPIOs")
> > > > >> >>
> > > > >> >>
> > > > >> >> [    1.951191] user_verbs: couldn't register device number
> > > > >> >
> > > > >> > Looks like user_verbs is using a static device node setup.
> > > > >> >
> > > > >> > enum {
> > > > >> >         IB_UVERBS_MAJOR       = 231,
> > > > >> >         IB_UVERBS_BASE_MINOR  = 192,
> > > > >> >         IB_UVERBS_MAX_DEVICES = 32
> > > > >> > };
> > > > >> >
> > > > >> > #define IB_UVERBS_BASE_DEV      MKDEV(IB_UVERBS_MAJOR, IB_UVERBS_BASE_MINOR)
> > > > >>
> > > > >> That's annoying...
> > > > >> I notice that infiniband is using register_chrdev_region() at
> > > > >> module_init() time, counting on device major 231 to be free.
> > > > >
> > > > > That device major is assigned to Infiniband, why shouldn't it be doing
> > > > > this?
> > > > 
> > > > I mean it's annoying that they collide. (Because of the details I
> > > > write below, it's fine it's using the assigned number.
> > > > 
> > > > > Why not just ask for a new reserved one?  We could give you 261 and
> > > > > everything should be fine, right?
> > > > 
> > > > Sure I can post a patch for that, but it just mitigates the problem.
> > > > 
> > > > The report point to the serious problem that on this system
> > > > some dynamic allocations have already stolen major device
> > > > numbers 232 thru 255, and 232 and 233 are also assigned.
> > > > 
> > > > What do you think about a patch that makes fs/char_dev.c
> > > > emit a warning when it starts assigning dynamic numbers
> > > > 233 and below?
> > > 
> > > That's fine with me.  I also think maybe we should look into just
> > > switching all char major/minor allocation to be dynamic, starting at the
> > > bottom and moving up.  I think the only tools that might have an issue
> > > with that is the raw device controller, but maybe that has been fixed up
> > > in userspace, I haven't looked at that in many years.
> > >
> > 
> > Is there any reason for the CHRDEV_MAJOR_HASH_SIZE being 255?
> > If we increase the size to say 511 will it break userspace?
> 
> No, that's an internal thing, but I don't see what that has to do with
> this...
> 
> > In the future I see a robot building a kernel with more that 255 devices and
> > having to deal with this kind of collision again.
> 
> We handle major numbers larger than 255 already...
> 
> > The handling of large major assignment baffles me.
> 
> It's tricky, messy, and something you don't want to touch, seriously...
>

Doh... I see now that each entry in the table is a list.

But then the dynamic allocation only looks for a NULL entry in the table.
So the limitation appears to be 255 dynamically allocated devices so still
has something to do with this.

Another simpler alternate to going completely dynamic is allocate dynamically
starting at an offset outside of the fixed range and just increment the major
on a first come first serve basis.

I guess that the current number would have to be stored somewhere.

Does this make sense or am I still not understanding how this works?

> good luck,
> 
> greg k-h