On Tue, Nov 22, 2022 at 06:08:00PM +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_PFNMAP, 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. > > Motivated by a recent patch which wanted to swich the system dma-buf > heap to vm_insert_page instead of vm_insert_pfn. > > v2: > > Jason brought up that we also want to guarantee that all ptes have the > pte_special flag set, to catch fast get_user_pages (on architectures > that support this). Allowing VM_MIXEDMAP (like VM_SPECIAL does) would > still allow vm_insert_page, but limiting to VM_PFNMAP will catch that. > > From auditing the various functions to insert pfn pte entires > (vm_insert_pfn_prot, remap_pfn_range and all it's callers like > dma_mmap_wc) it looks like VM_PFNMAP is already required anyway, so > this should be the correct flag to check for. I didn't look at how this actually gets used, but it is a bit of a pain to insert a lifetime controlled object like a struct page as a special PTE/VM_PFNMAP How is the lifetime model implemented here? How do you know when userspace has finally unmapped the page? Jason
