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

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Message-ID: <365c1ec2-cda6-4d94-895c-b2a795101857@redhat.com>
Date: Mon, 8 Sep 2025 17:46:00 +0200
From: David Hildenbrand <david@...hat.com>
To: 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 17:33, Jason Gunthorpe wrote:
> On Mon, Sep 08, 2025 at 05:24:23PM +0200, David Hildenbrand wrote:
>>>
>>>> I think we need to be cautious of scope here :) I don't want to
>>>> accidentally break things this way.
>>>
>>> IMHO it is worth doing when you get into more driver places it is far
>>> more obvious why the VM_SHARED is being checked.
>>>
>>>> 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'm thinking to call it vma_desc_never_cowable() as that is much much
>>> clear what the purpose is.
>>
>> Secretmem wants no private mappings. So we should check exactly that, not
>> whether we might have a cow mapping.
> 
> secretmem is checking shared for a different reason than many other places..

I think many cases just don't want any private mappings.

After all, you need a R/O file (VM_MAYWRITE cleared) mapped MAP_PRIVATE 
to make is_cow_mapping() == false.

And at that point, you just mostly have a R/O MAP_SHARED mapping IIRC.

-- 
Cheers

David / dhildenb