linux-kernel - Re: [PATCH v2 2/4] mm: fail get_vaddr_frames() for filesystem-dax mappings

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Date:   Mon, 27 Nov 2017 17:15:11 +0100
From:   Jan Kara <jack@...e.cz>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     akpm@...ux-foundation.org, Jan Kara <jack@...e.cz>,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        linux-nvdimm@...ts.01.org, Seung-Woo Kim <sw0312.kim@...sung.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Inki Dae <inki.dae@...sung.com>, linux-mm@...ck.org,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Mel Gorman <mgorman@...e.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-media@...r.kernel.org
Subject: Re: [PATCH v2 2/4] mm: fail get_vaddr_frames() for filesystem-dax
 mappings

On Tue 14-11-17 11:56:39, Dan Williams wrote:
> Until there is a solution to the dma-to-dax vs truncate problem it is
> not safe to allow V4L2, Exynos, and other frame vector users to create
> long standing / irrevocable memory registrations against filesytem-dax
> vmas.
> 
> Cc: Inki Dae <inki.dae@...sung.com>
> Cc: Seung-Woo Kim <sw0312.kim@...sung.com>
> Cc: Joonyoung Shim <jy0922.shim@...sung.com>
> Cc: Kyungmin Park <kyungmin.park@...sung.com>
> Cc: Mauro Carvalho Chehab <mchehab@...nel.org>
> Cc: linux-media@...r.kernel.org
> Cc: Jan Kara <jack@...e.cz>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: <stable@...r.kernel.org>
> Fixes: 3565fce3a659 ("mm, x86: get_user_pages() for dax mappings")
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>

Makes sense. I'd just note that in principle get_vaddr_frames() is no more
long-term than get_user_pages(). It is just so that all the users of
get_vaddr_frames() currently want a long-term reference. Maybe could you
add here also a comment that the vma_is_fsdax() check is there because all
users of this function want a long term page reference? With that you can
add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  mm/frame_vector.c |    4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/frame_vector.c b/mm/frame_vector.c
> index 72ebec18629c..d2fdbeaadc8b 100644
> --- a/mm/frame_vector.c
> +++ b/mm/frame_vector.c
> @@ -52,6 +52,10 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
>  		ret = -EFAULT;
>  		goto out;
>  	}
> +
> +	if (vma_is_fsdax(vma))
> +		return -EOPNOTSUPP;
> +
>  	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {
>  		vec->got_ref = true;
>  		vec->is_pfns = false;
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR