[PATCH 07/13] mm: remove the page_shift member from struct hmm_range
hch at lst.de
Thu Aug 1 06:49:14 UTC 2019
On Tue, Jul 30, 2019 at 05:50:16PM +0000, Jason Gunthorpe wrote:
> The way ODP seems to work is once in hugetlb mode the dma addresses
> must give huge pages or the page fault will be failed. I think that is
> a terrible design, but this is how the driver is ..
> So, from this HMM perspective if the caller asked for huge pages then
> the results have to be all huge pages or a hard failure.
Which isn't how the page_shift member works at moment. It still
allows non-hugetlb mappings even with the member.
> It is not negotiated as an optimization like you are thinking.
> [note, I haven't yet checked carefully how this works in ODP, every
> time I look at parts of it the thing seems crazy]
This seems pretty crazy. Especially as hugetlb use in applications
seems to fade in favour of THP, for which this ODP scheme does not seem
to work at all.
> > The best API for mlx4 would of course be to pass a biovec-style
> > variable length structure that hmm_fault could fill out, but that would
> > be a major restructure.
> It would work, but the driver has to expand that into a page list
> right awayhow.
> We can't even dma map the biovec with today's dma API as it needs the
> ability to remap on a page granularity.
We can do dma_map_page loops over each biovec entry pretty trivially,
and that won't be any worse than the current loop over each page in
the hmm dma helpers. Once I get around the work to have a better
API for iommu mappings for bio_vecs we could coalesce it similar to
how we do it with scatterlist (but without all the mess of a new
structure). That work is going to take a little longer through, as
it needs the amd and intell iommu drivers to be convered to dma-iommu
which isn't making progress as far as I hoped.
Let me know if you want to keep this code for now despite the issues,
or if we'd rather reimplement it once you've made sense of the ODP
More information about the amd-gfx