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Date:   Wed, 16 Nov 2022 11:50:31 +0100
From:   Daniel Vetter <daniel@ffwll.ch>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-ia64@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        dri-devel@lists.freedesktop.org, Hans Verkuil <hverkuil@xs4all.nl>,
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        Mike Kravetz <mike.kravetz@oracle.com>
Subject: Re: [PATCH mm-unstable v1 16/20] mm/frame-vector: remove FOLL_FORCE
 usage
Message-ID: <Y3TAd14jCBSKeqsL@phenom.ffwll.local>
Mail-Followup-To: David Hildenbrand <david@redhat.com>,
        linux-kernel@vger.kernel.org, linux-ia64@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        dri-devel@lists.freedesktop.org, Hans Verkuil <hverkuil@xs4all.nl>,
        linux-mm@kvack.org, Nadav Amit <namit@vmware.com>,
        linux-kselftest@vger.kernel.org, sparclinux@vger.kernel.org,
        Shuah Khan <shuah@kernel.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        linux-samsung-soc@vger.kernel.org, linux-rdma@vger.kernel.org,
        x86@kernel.org, Hugh Dickins <hughd@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Christoph Hellwig <hch@infradead.org>,
        Jason Gunthorpe <jgg@ziepe.ca>, Vlastimil Babka <vbabka@suse.cz>,
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        Alex Williamson <alex.williamson@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org,
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        linux-fsdevel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
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        Mike Kravetz <mike.kravetz@oracle.com>
References: <20221116102659.70287-1-david@redhat.com>
 <20221116102659.70287-17-david@redhat.com>
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On Wed, Nov 16, 2022 at 11:26:55AM +0100, David Hildenbrand wrote:
> FOLL_FORCE is really only for ptrace access. According to commit
> 707947247e95 ("media: videobuf2-vmalloc: get_userptr: buffers are always
> writable"), get_vaddr_frames() currently pins all pages writable as a
> workaround for issues with read-only buffers.
> 
> FOLL_FORCE, however, seems to be a legacy leftover as it predates
> commit 707947247e95 ("media: videobuf2-vmalloc: get_userptr: buffers are
> always writable"). Let's just remove it.
> 
> Once the read-only buffer issue has been resolved, FOLL_WRITE could
> again be set depending on the DMA direction.
> 
> Cc: Hans Verkuil <hverkuil@xs4all.nl>
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Tomasz Figa <tfiga@chromium.org>
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Also code I looked at while looking at follow_pfn stuff

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> ---
>  drivers/media/common/videobuf2/frame_vector.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/media/common/videobuf2/frame_vector.c b/drivers/media/common/videobuf2/frame_vector.c
> index 542dde9d2609..062e98148c53 100644
> --- a/drivers/media/common/videobuf2/frame_vector.c
> +++ b/drivers/media/common/videobuf2/frame_vector.c
> @@ -50,7 +50,7 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
>  	start = untagged_addr(start);
>  
>  	ret = pin_user_pages_fast(start, nr_frames,
> -				  FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM,
> +				  FOLL_WRITE | FOLL_LONGTERM,
>  				  (struct page **)(vec->ptrs));
>  	if (ret > 0) {
>  		vec->got_ref = true;
> -- 
> 2.38.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch