Re: User switchable HW mappings & cie

[Nick Piggin <npiggin@suse.de>]

On Mon, Oct 09, 2006 at 09:51:01PM +1000, Benjamin Herrenschmidt wrote:
> 
> > 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 ?

No, if you are MAP_SHARED then you shouldn't need to worry about that
indeed. For new code, I don't think it should be painful to avoid writing
to MAP_PRIVATE mappings.

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