linux-kernel - Re: [PATCH 03/16] mm: add vma_desc_size(), vma_desc

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Message-ID: <92def589-a76f-4360-8861-6bc9f94c1987@redhat.com>
Date: Mon, 8 Sep 2025 19:30:34 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Jason Gunthorpe <jgg@...dia.com>,
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Subject: Re: [PATCH 03/16] mm: add vma_desc_size(), vma_desc_pages() helpers

On 08.09.25 17:35, Lorenzo Stoakes wrote:
> On Mon, Sep 08, 2025 at 05:07:57PM +0200, David Hildenbrand wrote:
>> 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.
> 
> Right, but the vma_desc desribes a VMA being set up.
> 
> I mean is_cow_mapping(desc->vm_flags) isn't too egregious anyway, so maybe
> just use that for that case?

Yes, I don't think we would need another wrapper.

-- 
Cheers

David / dhildenb