On 15.06.22 09:54, Philippe Gerum wrote:
>
> Jan Kiszka via Xenomai <xenomai@xenomai.org> writes:
>
>> On 23.05.22 16:04, Gunter Grau via Xenomai wrote:
>>> From: Gunter Grau <gunter.g...@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.g...@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.
>
If Gunter is mapping IO memory, rtdm_mmap_to_user is surely not the
right thing.
Could it be that the prefault callback also has the side effect of
setting all page table entries that would otherwise only be filled lazily?
Jan
--
Siemens AG, Technology
Competence Center Embedded Linux
