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Message-ID: <36dd800b-d96b-af39-d0de-a5a8ca1034dd@xs4all.nl>
Date:   Wed, 23 Nov 2022 14:26:10 +0100
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To:     David Hildenbrand <david@redhat.com>,
        Tomasz Figa <tfiga@chromium.org>
Cc:     x86@kernel.org, linux-alpha@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org,
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        Linus Torvalds <torvalds@linux-foundation.org>,
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        Oded Gabbay <ogabbay@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Christoph Hellwig <hch@infradead.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
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References: <20221116102659.70287-1-david@redhat.com>
 <20221116102659.70287-17-david@redhat.com>
From:   Hans Verkuil <hverkuil@xs4all.nl>
Subject: Re: [PATCH mm-unstable v1 16/20] mm/frame-vector: remove FOLL_FORCE
 usage
In-Reply-To: <20221116102659.70287-17-david@redhat.com>
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Hi David, Tomasz,

On 16/11/2022 11:26, 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.

I've decided to revert 707947247e95: I have not been able to reproduce the problem
described in that commit, and Tomasz reported that it caused problems with a
specific use-case they encountered. I'll post that patch soon and I expect it
to land in 6.2. It will cause a conflict with this patch, though.

If the problem described in that patch occurs again, then I will revisit it
and hopefully do a better job than I did before. That commit was not my
finest moment.

Regards,

	Hans

> 
> 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>
> ---
>  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;