Re: [PATCH v2 8/8] mm/hmm: Indicate that HMM requires DMA coherency

Jason Gunthorpe
2026-03-12T12:35:15Z

On Wed, Mar 11, 2026 at 09:08:51PM +0200, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@nvidia.com>
>
> HMM mirroring can work on coherent systems without SWIOTLB path only.
> Until introduction of DMA_ATTR_REQUIRE_COHERENT, there was no reliable
> way to indicate that and various approximation was done:

HMM is fundamentally about allowing a sophisticated device to
independently DMA to a process's memory concurrently with the CPU
accessing the same memory. It is similar to SVA but does not rely on
IOMMU support. Since the entire use model is concurrent access to the
same memory it becomes fatally broken as a uAPI if SWIOTLB is
replacing the memory, or the CPU caches are incoherent with DMA.

Till now there was no reliable way to indicate that and various
approximation was done:

> int hmm_dma_map_alloc(struct device *dev, struct hmm_dma_map *map,
> size_t nr_entries, size_t dma_entry_size)
> {
> <...>
> /*
> * The HMM API violates our normal DMA buffer ownership rules and can't
> * transfer buffer ownership. The dma_addressing_limited() check is a
> * best approximation to ensure no swiotlb buffering happens.
> */
> dma_need_sync = !dev->dma_skip_sync;
> if (dma_need_sync || dma_addressing_limited(dev))
> return -EOPNOTSUPP;

Can it get dropped now then?

> So let's mark mapped buffers with DMA_ATTR_REQUIRE_COHERENT attribute
> to prevent DMA debugging warnings for cache overlapped entries.

Well, that isn't the main motivation, this prevents silent data
corruption if someone tries to use hmm in a system with swiotlb or
incoherent DMA,

Looks OK otherwise

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason