linux-kernel - Re: [PATCH 15/52] fuse: map virtio

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Message-ID: <CAA9_cmdLvFqG_Us5ZoBopj2ssWzxVbyNqf4XcgmY+=D_gARWDA@mail.gmail.com>
Date:   Thu, 13 Dec 2018 11:55:47 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     vgoyal@...hat.com
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>,
        Miklos Szeredi <miklos@...redi.hu>, stefanha@...hat.com,
        dgilbert@...hat.com, sweil@...hat.com, swhiteho@...hat.com
Subject: Re: [PATCH 15/52] fuse: map virtio_fs DAX window BAR

On Mon, Dec 10, 2018 at 9:22 AM Vivek Goyal <vgoyal@...hat.com> wrote:
>
> From: Stefan Hajnoczi <stefanha@...hat.com>
>
> Experimental QEMU code introduces an MMIO BAR for mapping portions of
> files in the virtio-fs device.  Map this BAR so that FUSE DAX can access
> file contents from the host page cache.

FUSE DAX sounds terrifying, can you explain a bit more about what this is?

> The DAX window is accessed by the fs/dax.c infrastructure and must have
> struct pages (at least on x86).  Use devm_memremap_pages() to map the
> DAX window PCI BAR and allocate struct page.

PCI BAR space is not cache coherent, what prevents these pages from
being used in paths that would do:

   object = page_address(pfn_to_page(virtio_fs_pfn));

...?