Messages in this thread | | | Date | Sun, 4 Oct 2020 09:50:59 -0300 | From | Jason Gunthorpe <> | Subject | Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM |
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On Sat, Oct 03, 2020 at 11:40:22AM +0200, Daniel Vetter wrote:
> > That leaves the only interesting places as vb2_dc_get_userptr() and > > vb2_vmalloc_get_userptr() which both completely fail to follow the > > REQUIRED behavior in the function's comment about checking PTEs. It > > just DMA maps them. Badly broken. > > > > Guessing this hackery is for some embedded P2P DMA transfer? > > Yeah, see also the follow_pfn trickery in > videobuf_dma_contig_user_get(), I think this is fully intentional and > userspace abi we can't break :-/
We don't need to break uABI, it just needs to work properly in the kernel:
vma = find_vma_intersection() dma_buf = dma_buf_get_from_vma(vma) sg = dma_buf_p2p_dma_map(dma_buf) [.. do dma ..] dma_buf_unmap(sg) dma_buf_put(dma_buf)
It is as we discussed before, dma buf needs to be discoverable from a VMA, at least for users doing this kind of stuff.
> Yup this should be done with dma_buf instead, and v4l has that. But > old uapi and all that. This is why I said we might need a new > VM_DYNAMIC_PFNMAP or so, to make follow_pfn not resolve this in the > case where the driver manages the underlying iomem range (or whatever > it is) dynamically and moves buffer objects around, like drm drivers > do. But I looked, and we've run out of vma->vm_flags :-(
A VM flag doesn't help - we need to introduce some kind of lifetime, and that has to be derived from the VMA. It needs data not just a flag
> The other problem is that I also have no real working clue about all > the VM_* flags and what they all mean, and whether drm drivers set the > right ones in all cases (they probably don't, but oh well). > Documentation for this stuff in headers is a bit thin at times.
Yah, I don't really know either :\
The comment above vm_normal_page() is a bit helpful. Don't know what VM_IO/VM_PFNMAP mean in their 3 combinations
There are very few places that set VM_PFNMAP without VM_IO..
Jason
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