linux-kernel - Re: [PATCH RFC 2/3] mm: Add support for exposing if dev

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Date:   Mon, 3 Dec 2018 11:47:37 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     alexander.h.duyck@...ux.intel.com
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Zhang Yi <yi.z.zhang@...ux.intel.com>,
        Barret Rhoden <brho@...gle.com>,
        KVM list <kvm@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Dave Jiang <dave.jiang@...el.com>,
        "Zhang, Yu C" <yu.c.zhang@...el.com>,
        Pankaj Gupta <pagupta@...hat.com>,
        David Hildenbrand <david@...hat.com>, Jan Kara <jack@...e.cz>,
        Christoph Hellwig <hch@....de>, rkrcmar@...hat.com,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [PATCH RFC 2/3] mm: Add support for exposing if dev_pagemap
 supports refcount pinning

On Mon, Dec 3, 2018 at 11:25 AM Alexander Duyck
<alexander.h.duyck@...ux.intel.com> wrote:
>
> Add a means of exposing if a pagemap supports refcount pinning. I am doing
> this to expose if a given pagemap has backing struct pages that will allow
> for the reference count of the page to be incremented to lock the page
> into place.
>
> The KVM code already has several spots where it was trying to use a
> pfn_valid check combined with a PageReserved check to determien if it could
> take a reference on the page. I am adding this check so in the case of the
> page having the reserved flag checked we can check the pagemap for the page
> to determine if we might fall into the special DAX case.
>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> ---
>  drivers/nvdimm/pfn_devs.c |    2 ++
>  include/linux/memremap.h  |    5 ++++-
>  include/linux/mm.h        |   11 +++++++++++
>  3 files changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/nvdimm/pfn_devs.c b/drivers/nvdimm/pfn_devs.c
> index 6f22272e8d80..7a4a85bcf7f4 100644
> --- a/drivers/nvdimm/pfn_devs.c
> +++ b/drivers/nvdimm/pfn_devs.c
> @@ -640,6 +640,8 @@ static int __nvdimm_setup_pfn(struct nd_pfn *nd_pfn, struct dev_pagemap *pgmap)
>         } else
>                 return -ENXIO;
>
> +       pgmap->support_refcount_pinning = true;
> +

There should be no dev_pagemap instance instance where this isn't
true, so I'm missing why this is needed?