Messages in this thread | | | Date | Mon, 9 Oct 2006 14:00:55 +0200 | From | Nick Piggin <> | Subject | Re: User switchable HW mappings & cie |
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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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