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SubjectRe: DMA mappings and crossing boundaries
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On Wed, 2018-07-04 at 13:57 +0100, Robin Murphy wrote:
>
> > Could it ? I wouldn't think dma_map_page is allows to cross page
> > boundaries ... what about single() ? The main worry is people using
> > these things on kmalloc'ed memory
>
> Oh, absolutely - the underlying operation is just "prepare for DMA
> to/from this physically-contiguous region"; the only real difference
> between map_page and map_single is for the sake of the usual "might be
> highmem" vs. "definitely lowmem" dichotomy. Nobody's policing any limits
> on the size and offset parameters (in fact, if anyone asks I would say
> the outcome of the big "offset > PAGE_SIZE" debate for dma_map_sg a few
> months back is valid for dma_map_page too, however silly it may seem).

I think this is a very bad idea though. We should thrive to avoid
coalescing before the iommu layer and we should avoid creating sglists
that cross natural alignment boundaries.

I had a look at sg_alloc_table_from_pages() and it basically freaks me
out.

Back in the old days, we used to have the block layer aggressively
coalesce requests iirc, but that was changed to let the iommu layer do
it.

If you pass to dma_map_sg() something already heavily coalesced, such
as what sg_alloc_table_from_pages() can do since it seems to have
absolutely no limits there, you will create a significant fragmentation
problem in the iommu allocator.

Instead, we should probably avoid such coalescing at that level and
instead let the iommu opportunistically coalesce at the point of
mapping which it does just fine.

We've been racking our brains here and we can't find a way to implement
something we want that is both very performance efficient (no global
locks etc...) and works with boundary crossing mappings.

We could always fallback to classic small page mappings but that is
quite suboptimal.

I also notice that sg_alloc_table_from_pages() doesn't try to prevent
crossing the 4G boundary. I remember pretty clearly that it was
explicitely forbidden to create such a crossing.

> Of course, given that the allocators tend to give out size/order-aligned
> chunks, I think you'd have to be pretty tricksy to get two allocations
> to line up either side of a large power-of-two boundary *and* go out of
> your way to then make a single request spanning both, but it's certainly
> not illegal. Realistically, the kind of "scrape together a large buffer
> from smaller pieces" code which is liable to hit a boundary-crossing
> case by sheer chance is almost certainly going to be taking the
> sg_alloc_table_from_pages() + dma_map_sg() route for convenience, rather
> than implementing its own merging and piecemeal mapping.

Yes and I think what sg_alloc_table_from_pages() does is quite wrong.

> > > Conceptually it looks pretty easy to extend the allocation constraints
> > > to cope with that - even the pathological worst case would have an
> > > absolute upper bound of 3 IOMMU entries for any one physical region -
> > > but if in practice it's a case of mapping arbitrary CPU pages to 32-bit
> > > DMA addresses having only 4 1GB slots to play with, I can't really see a
> > > way to make that practical :(
> >
> > No we are talking about 40-ish-bits of address space, so there's a bit
> > of leeway. Of course no scheme will work if the user app tries to map
> > more than the GPU can possibly access.
> >
> > But with newer AMD adding a few more bits and nVidia being at 47-bits,
> > I think we have some margin, it's just that they can't reach our
> > discontiguous memory with a normal 'bypass' mapping and I'd rather not
> > teach Linux about every single way our HW can scatter memory accross
> > nodes, so an "on demand" mechanism is by far the most flexible way to
> > deal with all configurations.
> >
> > > Maybe the best compromise would be some sort of hybrid scheme which
> > > makes sure that one of the IOMMU entries always covers the SWIOTLB
> > > buffer, and invokes software bouncing for the awkward cases.
> >
> > Hrm... not too sure about that. I'm happy to limit that scheme to well
> > known GPU vendor/device IDs, and SW bouncing is pointless in these
> > cases. It would be nice if we could have some kind of guarantee that a
> > single mapping or sglist entry never crossed a specific boundary
> > though... We more/less have that for 4G already (well, we are supposed
> > to at least). Who are the main potential problematic subsystems here ?
> > I'm thinking network skb allocation pools ... and page cache if it
> > tries to coalesce entries before issuing the map request, does it ?
>
> I don't know of anything definite off-hand, but my hunch is to be most
> wary of anything wanting to do zero-copy access to large buffers in
> userspace pages. In particular, sg_alloc_table_from_pages() lacks any
> kind of boundary enforcement (and most all users don't even use the
> segment-length-limiting variant either), so I'd say any caller of that
> currently has a very small, but nonzero, probability of spoiling your day.

And I'm starting to think we should just fix them.

Cheers,
Ben.

> Robin.
>

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