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SubjectRe: [PATCH v3 2/2] mm, drm/ttm: Fix vm page protection handling
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Hi Michal,

On Fri, 2019-12-06 at 11:30 +0100, Michal Hocko wrote:
> On Fri 06-12-19 09:24:26, Thomas Hellström (VMware) wrote:
> [...]
> > @@ -283,11 +282,26 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct
> > vm_fault *vmf,
> > pfn = page_to_pfn(page);
> > }
> >
> > + /*
> > + * Note that the value of @prot at this point may
> > differ from
> > + * the value of @vma->vm_page_prot in the caching- and
> > + * encryption bits. This is because the exact location
> > of the
> > + * data may not be known at mmap() time and may also
> > change
> > + * at arbitrary times while the data is mmap'ed.
> > + * This is ok as long as @vma->vm_page_prot is not used
> > by
> > + * the core vm to set caching- and encryption bits.
> > + * This is ensured by core vm using pte_modify() to
> > modify
> > + * page table entry protection bits (that function
> > preserves
> > + * old caching- and encryption bits), and the @fault
> > + * callback being the only function that creates new
> > + * page table entries.
> > + */
>
> While this is a very valuable piece of information I believe we need
> to
> document this in the generic code where everybody will find it.
> vmf_insert_mixed_prot sounds like a good place to me. So being
> explicit
> about VM_MIXEDMAP. Also a reference from vm_page_prot to this
> function
> would be really helpeful.
>
> Thanks!
>

Just to make sure I understand correctly. You'd prefer this (or
similar) text to be present at the vmf_insert_mixed_prot() and
vmf_insert_pfn_prot() definitions for MIXEDMAP and PFNMAP respectively,
and a pointer from vm_page_prot to that text. Is that correct?

Thanks,
Thomas


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