linux-kernel - Re: [PATCH v10 1/8] mm: introduce FOLL_PCI_P2PDMA to gate getting PCI P2PDMA pages

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Date:   Fri, 23 Sep 2022 15:13:01 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, Christoph Hellwig <hch@....de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Martin Oliveira <martin.oliveira@...eticom.com>,
        Chaitanya Kulkarni <ckulkarnilinux@...il.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [PATCH v10 1/8] mm: introduce FOLL_PCI_P2PDMA to gate getting
 PCI P2PDMA pages

On Thu, Sep 22, 2022 at 10:39:19AM -0600, Logan Gunthorpe wrote:
> GUP Callers that expect PCI P2PDMA pages can now set FOLL_PCI_P2PDMA to
> allow obtaining P2PDMA pages. If GUP is called without the flag and a
> P2PDMA page is found, it will return an error.
> 
> FOLL_PCI_P2PDMA cannot be set if FOLL_LONGTERM is set.

What is causing this? It is really troublesome, I would like to fix
it. eg I would like to have P2PDMA pages in VFIO iommu page tables and
in RDMA MR's - both require longterm.

Is it just because ZONE_DEVICE was created for DAX and carried that
revocable assumption over? Does anything in your series require
revocable?

> @@ -2383,6 +2392,10 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>  		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
>  		page = pte_page(pte);
>  
> +		if (unlikely(!(flags & FOLL_PCI_P2PDMA) &&
> +			     is_pci_p2pdma_page(page)))
> +			goto pte_unmap;
> +
>  		folio = try_grab_folio(page, 1, flags);
>  		if (!folio)
>  			goto pte_unmap;

On closer look this is not in the right place, we cannot touch the
content of *page without holding a ref, and that doesn't happen until
until try_grab_folio() completes.

It would be simpler to put this check in try_grab_folio/try_grab_page
after the ref has been obtained. That will naturally cover all the
places that need it.

Jason