Re: [PATCH 03/16] mm: add vma_desc_size(), vma_desc_pages() helpers

David Hildenbrand
2025-09-08T15:08:04Z

On 08.09.25 16:47, Lorenzo Stoakes wrote:
> On Mon, Sep 08, 2025 at 11:20:11AM -0300, Jason Gunthorpe wrote:
>> On Mon, Sep 08, 2025 at 03:09:43PM +0100, Lorenzo Stoakes wrote:
>>>> Perhaps
>>>>
>>>> !vma_desc_cowable()
>>>>
>>>> Is what many drivers are really trying to assert.
>>>
>>> Well no, because:
>>>
>>> static inline bool is_cow_mapping(vm_flags_t flags)
>>> {
>>> return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
>>> }
>>>
>>> Read-only means !CoW.
>>
>> What drivers want when they check SHARED is to prevent COW. It is COW
>> that causes problems for whatever the driver is doing, so calling the
>> helper cowable and making the test actually right for is a good thing.
>>
>> COW of this VMA, and no possibilty to remap/mprotect/fork/etc it into
>> something that is COW in future.
>
> But you can't do that if !VM_MAYWRITE.
>
> I mean probably the driver's just wrong and should use is_cow_mapping() tbh.
>
>>
>> Drivers have commonly various things with VM_SHARED to establish !COW,
>> but if that isn't actually right then lets fix it to be clear and
>> correct.
>
> I think we need to be cautious of scope here :) I don't want to accidentally
> break things this way.
>
> OK I think a sensible way forward - How about I add desc_is_cowable() or
> vma_desc_cowable() and only set this if I'm confident it's correct?

I'll note that the naming is bad.

Why?

Because the vma_desc is not cowable. The underlying mapping maybe is.

--
Cheers

David / dhildenb