[PATCH v5 24/38] mm/vma: enforce that mm-managed mappings may not set VMA_IO_BIT

[David Hildenbrand (Arm)]

On 10/12/26 20:35, Lorenzo Stoakes (ARM) wrote:
> On Mon, Oct 12, 2026 at 08:25:54PM +0200, David Hildenbrand (Arm) wrote:
>> On 10/9/26 20:04, Lorenzo Stoakes (ARM) wrote:
>>> It makes no sense for a mapping whose contents the kernel does not own to
>>> specify that the range is MMIO.
>>>
>>> Prior to this patch, all in-tree drivers which did so have been updated
>>> such that they are marked as not being mm-managed. The check WARNs and
>>> fails the mmap for any out-of-tree driver that still sets VMA_IO_BIT
>>> without a kernel mapping.
>>>
>>> No functional change intended for in-tree code.
>>>
>>> Reviewed-by: Zi Yan <ziy at nvidia.com>
>>> Signed-off-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
>>> ---
>>>  mm/vma.c | 6 ++++++
>>>  1 file changed, 6 insertions(+)
>>>
>>> diff --git a/mm/vma.c b/mm/vma.c
>>> index 4f09fb98a2fd..51bdbe08bb02 100644
>>> --- a/mm/vma.c
>>> +++ b/mm/vma.c
>>> @@ -2796,6 +2796,12 @@ static int mmap_validate_vma_flags(const vma_flags_t *flags)
>>>  		return -EINVAL;
>>>  #endif
>>>
>>> +	if (!vma_flags_test_any(flags, VMA_PFNMAP_BIT, VMA_MIXEDMAP_BIT)) {
>>> +		/* mm-managed mappings may not set VMA_IO_BIT. */
>>> +		if (WARN_ON_ONCE(vma_flags_test(flags, VMA_IO_BIT)))
>>> +			return -EINVAL;
>>
>> We do have a helper (vma_flags_is_mm_managed()) but we're not using it because
>> of the VM_IO situation ...
> 
> That gets removed.
> 
>>
>> That's confusing ...
>>
>> Could we do something like testing vma_flags_is_mm_managed(), but removing the
>> VM_IO flag from the checked flags?
> 
> I mean we do that in a subsequent commit, I think even the next one?
> 
> 
>>
>> Then we can add a comment about VM_IO in the future getting remove from
>> vma_flags_is_mm_managed() ...
> 
> Yeah, but to do that the code would now be something like:
> 
> 	vma_flags_t masked = flags;
> 
> 	/*
> 	  * vma_flags_is_mm_managed() tests VMA_IO_BIT, which will be removed
> 	 * in the next commit.
> 	 */
> 	vma_flags_clear(&flags, VMA_IO_BIT);
> 	if (!vma_flags_is_mm_managed(masked)) {
> 		...
> 	}
> 
> Is that really worth it?
> 
> I don't think there's really too much confusion intra-patch about that is there?

I would do it as above (- the comment, as that is the next patch indeed) and
only remove the vma_flags_clear() in the next patch.

But in any case, you should spell out that this is only temporary in the patch
description :)

-- 
Cheers,

David