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

Lorenzo Stoakes
2025-09-08T15:36:28Z

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?

>
> --
> Cheers
>
> David / dhildenb
>

Cheers, Lorenzo