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. -- Best regards, Dmitry
