Re: [PATCH v1 00/16] dma-mapping: migrate to physical address-based API

[Demi Marie Obenour <demiobenour@xxxxxxxxx>]

On 8/9/25 09:34, Jason Gunthorpe wrote:
> On Fri, Aug 08, 2025 at 08:51:08PM +0200, Marek Szyprowski wrote:
>> First - basing the API on the phys_addr_t.
>>
>> Page based API had the advantage that it was really hard to abuse it and 
>> call for something that is not 'a normal RAM'. 
> 
> This is not true anymore. Today we have ZONE_DEVICE as a struct page
> type with a whole bunch of non-dram sub-types:
> 
> enum memory_type {
>       /* 0 is reserved to catch uninitialized type fields */
>       MEMORY_DEVICE_PRIVATE = 1,
>       MEMORY_DEVICE_COHERENT,
>       MEMORY_DEVICE_FS_DAX,
>       MEMORY_DEVICE_GENERIC,
>       MEMORY_DEVICE_PCI_P2PDMA,
> };
> 
> Few of which are kmappable/page_to_virtable() in a way that is useful
> for the DMA API.
> 
> DMA API sort of ignores all of this and relies on the caller to not
> pass in an incorrect struct page. eg we rely on things like the block
> stack to do the right stuff when a MEMORY_DEVICE_PCI_P2PDMA is present
> in a bio_vec.
> 
> Which is not really fundamentally different from just using
> phys_addr_t in the first place.
> 
> Sure, this was a stronger argument when this stuff was originally
> written, before ZONE_DEVICE was invented.
> 
>> I initially though that phys_addr_t based API will somehow simplify
>> arch specific implementation, as some of them indeed rely on
>> phys_addr_t internally, but I missed other things pointed by
>> Robin. Do we have here any alternative?
> 
> I think it is less of a code simplification, more as a reduction in
> conceptual load. When we can say directly there is no struct page type
> anyhwere in the DMA API layers then we only have to reason about
> kmap/phys_to_virt compatibly.
> 
> This is also a weaker overall requirement than needing an actual
> struct page which allows optimizing other parts of the kernel. Like we
> aren't forced to create MEMORY_DEVICE_PCI_P2PDMA stuct pages just to
> use the dma api.
> 
> Again, any place in the kernel we can get rid of struct page the
> smoother the road will be for the MM side struct page restructuring.
> 
> For example one of the bigger eventual goes here is to make a bio_vec
> store phys_addr_t, not struct page pointers.
> 
> DMA API is not alone here, we have been de-struct-paging the kernel
> for a long time now:
> 
> netdev: 
> https://lore.kernel.org/linux-mm/20250609043225.77229-1-byungchul@xxxxxx/
> slab: https://lore.kernel.org/linux-mm/20211201181510.18784-1-vbabka@xxxxxxx/
> iommmu: 
> https://lore.kernel.org/all/0-v4-c8663abbb606+3f7-iommu_pages_jgg@xxxxxxxxxx/
> page tables: 
> https://lore.kernel.org/linux-mm/20230731170332.69404-1-vishal.moola@xxxxxxxxx/
> zswap: 
> https://lore.kernel.org/all/20241216150450.1228021-1-42.hyeyoo@xxxxxxxxx/
> 
> With a long term goal that struct page only exists for legacy code,
> and is maybe entirely compiled out of modern server kernels.

Why just server kernels?  I suspect client systems actually run
newer kernels than servers do.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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