On 1/19/25 23:02, Dmitry Osipenko wrote: > On 1/19/25 19:18, Sasha Finkelstein wrote: >> On Sun, 19 Jan 2025 at 12:50, Dmitry Osipenko >> <dmitry.osipenko@xxxxxxxxxxxxx> wrote: >>>> ret = io_remap_pfn_range(vma, vma->vm_start, >>>> - vram->vram_node.start >> PAGE_SHIFT, >>>> + (vram->vram_node.start >> PAGE_SHIFT) + vma->vm_pgoff, >>>> vm_size, vma->vm_page_prot); >>>> return ret; >>>> } >>> >>> The vma->vm_pgoff is fake in DRM, it's used for looking up DRM GEM >>> object based on the vma->vm_pgoff value when mmap is invoked. >> >> If my understanding is correct, vm_pgoff gets "unfaked" by >> https://elixir.bootlin.com/linux/v6.12.6/source/drivers/gpu/drm/virtio/virtgpu_vram.c#L48 >> >>> vma->vm_pgoff should be treated as zero here. Hence we can map a part of >>> GEM, but only from its start. See drm_gem_mmap(). >> >> I've had a "v0" (not on ml) of this patch that always treated vma->vm_pgoff as >> zero. This broke when anything tried to mmap with a non-zero offset. Adding >> vm_pgoff made it work correctly. > > I've tested this patch. Partial mapping with a non-zero offset doesn't > work because drm_gem_mmap() rejects it. I'd want to see your sample code > that performs mmaping, maybe I'm missing something. > >>> Please correct vma->vm_pgoff in v2. >> >> I need apps to be able to mmap with a non-zero offset for my usecase. >> While the correct value may be something else other than what is in >> the current patch, 0 is definitely incorrect for at least some workloads. > > drm_gem_mmap() uses drm_vma_offset_exact_lookup_locked() that doesn't > allow vma->vm_pgoff != node.start. AFAICT, no one driver supports > mapping with a non-zero offset, perhaps for a reason that I don't know > about. See now that a non-zero mapping of a dmabuf might work. Will test it. -- Best regards, Dmitry
