On Wed, Feb 03, 2021 at 10:19:48PM +0100, Daniel Vetter wrote: > tldr; DMA buffers aren't normal memory, expecting that you can use > them like that (like calling get_user_pages works, or that they're > accounting like any other normal memory) cannot be guaranteed. > > Since some userspace only runs on integrated devices, where all > buffers are actually all resident system memory, there's a huge > temptation to assume that a struct page is always present and useable > like for any more pagecache backed mmap. This has the potential to > result in a uapi nightmare. > > To stop this gap require that DMA buffer mmaps are VM_SPECIAL, which > blocks get_user_pages and all the other struct page based > infrastructure for everyone. In spirit this is the uapi counterpart to > the kernel-internal CONFIG_DMABUF_DEBUG. Fast gup needs the special flag set on the PTE as well.. Feels weird to have a special VMA without also having special PTEs? Jason
