How to handle P2P DMA with only {physaddr,len} in bio_vec?

[David Howells <dhowells@redhat.com>]

Hi Christoph,

Looking at the DMA address mapping infrastructure, it makes use of the page
struct to access the physical address (which obviously shouldn't be a problem)
and to find out if the page is involved in P2P DMA.

dma_direct_map_page() calls is_pci_p2pdma_page():

	static inline bool is_pci_p2pdma_page(const struct page *page)
	{
		return IS_ENABLED(CONFIG_PCI_P2PDMA) &&
			is_zone_device_page(page) &&
			page_pgmap(page)->type == MEMORY_DEVICE_PCI_P2PDMA;
	}

What's the best way to manage this without having to go back to the page
struct for every DMA mapping we want to make?  Do we need to have
iov_extract_user_pages() note this in the bio_vec?

	struct bio_vec {
		physaddr_t	bv_base_addr;	/* 64-bits */
		size_t		bv_len:56;	/* Maybe just u32 */
		bool		p2pdma:1;	/* Region is involved in P2P */
		unsigned int	spare:7;
	};

I'm guessing that only folio-type pages can be involved in this:

	static inline struct dev_pagemap *page_pgmap(const struct page *page)
	{
		VM_WARN_ON_ONCE_PAGE(!is_zone_device_page(page), page);
		return page_folio(page)->pgmap;
	}

as only struct folio has a pointer to dev_pagemap?  And I assume this is going
to get removed from struct page itself at some point soonish.

David