Am 06.04.2018 um 11:33 schrieb Gerd Hoffmann:

The pages backing a DMA-buf are not allowed to move (at least not without a
patch set I'm currently working on), but for certain MM operations to work
correctly you must be able to modify the page tables entries and move the
pages backing them around.

For example try to use fork() with some copy on write pages with this
approach. You will find that you have only two options to correctly handle
The fork() issue should go away with shared memory pages (no cow).
I guess this is the reason why vgem is internally backed by shmem.

Yes, exactly that is also an approach which should work fine. Just don't try to get this working with get_user_pages().

Hmm.  So I could try to limit the udmabuf driver to shmem too (i.e.
have the ioctl take a shmem filehandle and offset instead of a virtual

But maybe it is better then to just extend vgem, i.e. add support to
create gem objects from existing shmem.


Yes, extending vgem instead of creating something new sounds like a good idea to me as well.



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