Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM

[Daniel Vetter]

On Mon, Oct 5, 2020 at 8:54 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
>
> On Mon, Oct 5, 2020 at 8:37 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> >
> > On Mon, Oct 05, 2020 at 08:16:33PM +0200, Daniel Vetter wrote:
> >
> > > > kvm is some similar hack added for P2P DMA, see commit
> > > > add6a0cd1c5ba51b201e1361b05a5df817083618. It might be protected by notifiers..
> > >
> > > Yeah my thinking is that kvm (and I think also vfio, also seems to
> > > have mmu notifier nearby) are ok because of the mmu notiifer. Assuming
> > > that one works correctly.
> >
> > vfio doesn't have a notifier, Alex was looking to add a vfio private
> > scheme in the vma->private_data:
> >
> > https://lore.kernel.org/kvm/159017449210.18853.15037950701494323009.stgit@xxxxxxxxxx/
> >
> > Guess it never happened.
>
> I was mislead by the mmu notifier in drivers/vfio/vfio.c. But looking
> closer, that's only used by some drivers, I guess to make sure their
> device pagetables are kept in sync with reality. And not to make sure
> the vfio pfn view is kept in sync with reality.
>
> This could get real nasty I think.
>
> > > > So, the answer really is that s390 and media need fixing, and this API
> > > > should go away (or become kvm specific)
> > >
> > > I'm still not clear how you want fo fix this, since your vma->dma_buf
> > > idea is kinda a decade long plan and so just not going to happen:
> >
> > Well, it doesn't mean we have to change every part of dma_buf to
> > participate in this. Just the bits media cares about. Or maybe it is
> > some higher level varient on top of dma_buf.
> >
> > Or don't use dma_buf for this, add a new object that just provides
> > refcounts and P2P DMA connection for IO pfn ranges..
>
> So good news is, I dug some layers deeper in v4l, and there's only 2
> users which do actually handle pfn and don't immediately convert to a
> pages array:
> - videbuf-dma-contig.c. Luckily videobuf 1 is deprecated since
> forever, so I think we might get away with either just breaking this,
> or at least tainting kernels and hiding it behind a nasty Kconfig.
> This only uses follow_pfn, which we need to keep anyway for vfio in
> the unsafe variant :-/
> - videbuf2-vmalloc.c Digging through history this was added to support
> import of v4l buffers from drivers that needed contig memory. And way
> back before CMA, that meant carveout memory not backed by struct page
> *. That should now all have struct pages and be managed by CMA (since
> videbuf2-dma-contig.c just uses dma_alloc_coherent underneath), so I
> think we can just switch to pin_user_pages(FOLL_LONGTERM here too).
>
> iow I think I can outright delete the frame vector stuff.

Ok this doesn't work, because dma_mmap always uses a remap_pfn_range,
which is a VM_IO | VM_PFNMAP vma and so even if it's cma backed and
not a carveout, we can't get the pages. Plus trying to move the cma
pages out of cma for FOLL_LONGTERM would be kinda bad when they've
been allocated as a contig block by dma_alloc_coherent :-)

So this idea of switching over to pup only is going to break zerocopy.
I guess I'll need something else for this then.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch