linux-kernel - Re: [PATCH v2] comedi: Flush partial mappings in error case

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Message-ID: <cf1a2818-1b78-4422-bb76-421732c428c0@mev.co.uk>
Date: Thu, 17 Oct 2024 10:48:10 +0100
From: Ian Abbott <abbotti@....co.uk>
To: Jann Horn <jannh@...gle.com>, kernel test robot <lkp@...el.com>
Cc: H Hartley Sweeten <hsweeten@...ionengravers.com>,
 Frank Mori Hess <fmh6jj@...il.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 oe-kbuild-all@...ts.linux.dev, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH v2] comedi: Flush partial mappings in error case

On 17/10/2024 10:29, Ian Abbott wrote:
> On 16/10/2024 23:05, Jann Horn wrote:
>> On Wed, Oct 16, 2024 at 8:05 PM kernel test robot <lkp@...el.com> wrote:
>>> [auto build test ERROR on 6485cf5ea253d40d507cd71253c9568c5470cd27]
>>>
>>> url:    https://github.com/intel-lab-lkp/linux/commits/Jann-Horn/ 
>>> comedi-Flush-partial-mappings-in-error-case/20241016-022809
>>> base:   6485cf5ea253d40d507cd71253c9568c5470cd27
>>> patch link:    https://lore.kernel.org/r/20241015-comedi-tlb-v2-1- 
>>> cafb0e27dd9a%40google.com
>>> patch subject: [PATCH v2] comedi: Flush partial mappings in error case
>>> config: arm-randconfig-004-20241016 (https://download.01.org/0day-ci/ 
>>> archive/20241017/202410170111.K30oyTWa-lkp@...el.com/config)
>>> compiler: arm-linux-gnueabi-gcc (GCC) 14.1.0
>>> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/ 
>>> archive/20241017/202410170111.K30oyTWa-lkp@...el.com/reproduce)
>>>
>>> If you fix the issue in a separate patch/commit (i.e. not just a new 
>>> version of
>>> the same patch/commit), kindly add following tags
>>> | Reported-by: kernel test robot <lkp@...el.com>
>>> | Closes: https://lore.kernel.org/oe-kbuild- 
>>> all/202410170111.K30oyTWa-lkp@...el.com/
>>>
>>> All errors (new ones prefixed by >>):
>>>
>>>     arm-linux-gnueabi-ld: drivers/comedi/comedi_fops.o: in function 
>>> `comedi_mmap':
>>>>> comedi_fops.c:(.text+0x4be): undefined reference to `zap_vma_ptes'
>>
>> Ugh, this one is from a nommu build ("# CONFIG_MMU is not set"), it
>> makes sense that you can't zap PTEs when you don't have any PTEs at
>> all... what really impresses me about this is that the rest of the
>> code compiles on nommu. I'm pretty sure this codepath wouldn't
>> actually _work_ on nommu, but apparently compiling it works?
>>
>> I don't know what the right fix is here - should the entire comedi
>> driver be gated on CONFIG_MMU, or only a subset of the mmap handler,
>> or something else?
> 
> Given that it would also affect a lot of fbdev drivers that would also 
> benefit from zapping partial mappings, I suggest that gating on 
> CONFIG_MMU would not be the correct fix.

Perhaps just add an #ifdef CONFIG_MMU around the affected call for now?

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