linux-kernel - How to handle P2P DMA with only {physaddr,len} in bio

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Message-ID: <1098395.1750675858@warthog.procyon.org.uk>
Date: Mon, 23 Jun 2025 11:50:58 +0100
From: David Howells <dhowells@...hat.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: dhowells@...hat.com, Andrew Lunn <andrew@...n.ch>,
    Eric Dumazet <edumazet@...gle.com>,
    "David S. Miller" <davem@...emloft.net>,
    Jakub Kicinski <kuba@...nel.org>,
    David Hildenbrand <david@...hat.com>,
    John Hubbard <jhubbard@...dia.com>,
    Mina Almasry <almasrymina@...gle.com>, willy@...radead.org,
    Christian Brauner <brauner@...nel.org>,
    Al Viro <viro@...iv.linux.org.uk>, netdev@...r.kernel.org,
    linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
    linux-kernel@...r.kernel.org
Subject: How to handle P2P DMA with only {physaddr,len} in bio_vec?

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