On Wed, Sep 16, 2020 at 1:45 PM Christian König <christian.koenig@xxxxxxx> wrote: > > [SNIP] > > But Jason pointed me to the right piece of code. See this comment in in mmap_region(): > > /* ->mmap() can change vma->vm_file, but must guarantee that > * vma_link() below can deny write-access if VM_DENYWRITE is set > * and map writably if VM_SHARED is set. This usually means the > * new file must not have been exposed to user-space, yet. > */ > vma->vm_file = get_file(file); > error = call_mmap(file, vma); > > > So changing vma->vm_file is allowed at least under certain circumstances. > > Only the "file must not have been exposed to user-space, yet" part still needs double checking. Currently working on that. > > > Ok, I think we can guarantee for all DMA-bufs what is required here. > > While searching the code I've found that at least vgem_prime_mmap() and i915_gem_dmabuf_mmap() are doing the same thing of modifying vma->vm_file. > > So I'm leaning towards that this works as expected and we should just document this properly. > > Daniel and Jason what do you think? Well I can claim I started this, so I started out with naively assuming that it Just Works :-) I think we already have vgem/i915 prime testcases in igt which try to excercise this mmap forwarding, including provoking pte shoot-downs. So I think best would be if you could also add a variant for amdgpu, to make sure this really works and keeps working. Plus ofc the documentation patch so that core mm folks can officially ack this as legit. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
