Re: [RFC PATCH 2/2] mm,drm/ttm: Use VM_PFNMAP for TTM vmas

[Jason Gunthorpe]

On Tue, Mar 23, 2021 at 12:47:24PM +0100, Daniel Vetter wrote:

> > +static inline bool is_cow_mapping(vm_flags_t flags)
> 
> Bit a bikeshed, but I wonder whether the public interface shouldn't be
> vma_is_cow_mapping. Or whether this shouldn't be rejected somewhere else,
> since at least in drivers/gpu we have tons of cases that don't check for
> this and get it all kinds of wrong I think.
> 
> remap_pfn_range handles this for many cases, but by far not for all.
> 
> Anyway patch itself lgtm:
> 
> Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>

I would like it if io_remap_pfn_range() did not allow shared mappings
at all.

IIRC it doesn't work anyway, the kernel can't reliably copy from IO
pages eg the "_copy_from_user_inatomic()" under cow_user_page() will
not work on s390 that requires all IO memory be accessed with special
instructions.

Unfortunately I have no idea what the long ago special case of
allowing COW'd IO mappings is. :\

Jason