Re: [PATCH] rtdm/drvlib: Prevent pagefaults on arm on io mapping

[Philippe Gerum <rpm@xenomai.org>]

Jan Kiszka via Xenomai <xenomai@xenomai.org> writes:

> On 23.05.22 16:04, Gunter Grau via Xenomai wrote:
>> From: Gunter Grau <gunter.grau@philips.com>
>> 
>> When mapping io memory into userspace an extra simulated pagefault for all
>> pages is added to prevent later pagefaults because of copy on write
>> mechanisms. This happens only on architectures that have defined the
>> needed cobalt_machine.prefault function.
>> 
>> Signed-off-by: Gunter Grau <gunter.grau@philips.com>
>> ---
>>  kernel/cobalt/rtdm/drvlib.c | 10 +++++++++-
>>  1 file changed, 9 insertions(+), 1 deletion(-)
>> 
>> diff --git a/kernel/cobalt/rtdm/drvlib.c b/kernel/cobalt/rtdm/drvlib.c
>> index 4eaf3a57c..db8431ee1 100644
>> --- a/kernel/cobalt/rtdm/drvlib.c
>> +++ b/kernel/cobalt/rtdm/drvlib.c
>> @@ -1761,6 +1761,7 @@ static int mmap_iomem_helper(struct vm_area_struct *vma, phys_addr_t pa)
>>  {
>>  	pgprot_t prot = PAGE_SHARED;
>>  	unsigned long len;
>> +	int ret;
>>  
>>  	len = vma->vm_end - vma->vm_start;
>>  #ifndef CONFIG_MMU
>> @@ -1774,8 +1775,15 @@ static int mmap_iomem_helper(struct vm_area_struct *vma, phys_addr_t pa)
>>  #endif
>>  	vma->vm_page_prot = pgprot_noncached(prot);
>>  
>> -	return remap_pfn_range(vma, vma->vm_start, pa >> PAGE_SHIFT,
>> +	ret = remap_pfn_range(vma, vma->vm_start, pa >> PAGE_SHIFT,
>>  			       len, vma->vm_page_prot);
>> +	if (ret)
>> +		return ret;
>> +
>> +	if (cobalt_machine.prefault)
>> +		cobalt_machine.prefault(vma);
>> +
>> +	return ret;
>>  }
>>  
>>  static int mmap_buffer_helper(struct rtdm_fd *fd, struct vm_area_struct *vma)
>
> Wow, that was likely broken by the refactoring in c8e9e166, long ago.
>
> Applied to next
>

The prefault hook has always been specifically about COW-breaking, I/O
memory has no business with this, so there is no point in having the
iomem helper calling the prefaulting hook.

I suspect that rtdm_mmap_to_user() should be called instead of
rtdm_iomap_to_user() in the case at hand.

-- 
Philippe.