Re: User switchable HW mappings & cie

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

> I'm very much for this approach, possibly with the extension that we 
> could have a multiple-page version as well, as populating the whole vma 
> sometimes may be cheaper than populating each pte with a fault. That 
> would basically be an io_remap_pfn_range() which is safe when the 
> mmap_sem is taken in read mode (from do_no_page).
> 
> One problem that occurs is that the rule for ptes with non-backing 
> struct pages
> Which I think was introduced in 2.6.16:
> 
>     pfn_of_page == vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT)
> 
> cannot be honored, at least not with the DRM memory manager, since the 
> graphics object will be associated with a vma and not the underlying 
> physical address. User space will have vma->vm_pgoff as a handle to the 
> object, which may move around in graphics memory.

That's a problem with VM_PFNMAP set indeed. get_user_pages() is a
non-issue with VM_IO set too but I'm not sure about other code path that
might try to hit here... though I think we don't hit that if MAP_SHARED,
Nick ?

Ben.


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