On Thu, Feb 4, 2021 at 5:13 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote: > 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? There's kinda no convenient & cheap way to check for the pte_special flag. This here should at least catch accidental misuse, people building their own ptes we can't stop. Maybe we should exclude VM_MIXEDMAP to catch vm_insert_page in one of these. Hm looking at code I think we need to require VM_PFNMAP here to stop vm_insert_page. And looking at the various functions, that seems to be required (and I guess VM_IO is more for really funky architectures where io-space is somewhere else?). I guess I should check for VM_PFNMAP instead of VM_SPECIAL? -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
