Re: [PATCH RFC 2/3] mm: Add support for exposing if dev_pagemap supports refcount pinning

[Dan Williams]

On Mon, Dec 3, 2018 at 11:25 AM Alexander Duyck
<alexander.h.duyck@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> ---
>  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?