linux-kernel - Re: [PATCH RFC 16/19] mm/frame-vector: remove FOLL

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Date:   Tue, 8 Nov 2022 13:45:57 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     David Hildenbrand <david@...hat.com>,
        Hans Verkuil <hverkuil@...all.nl>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        etnaviv@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-media@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        Peter Xu <peterx@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, Nadav Amit <namit@...are.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Lucas Stach <l.stach@...gutronix.de>,
        David Airlie <airlied@...il.com>,
        Oded Gabbay <ogabbay@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Subject: Re: [PATCH RFC 16/19] mm/frame-vector: remove FOLL_FORCE usage

Hi David,

On Tue, Nov 8, 2022 at 1:19 AM David Hildenbrand <david@...hat.com> wrote:
>
> FOLL_FORCE is really only for debugger access. According to commit
> 707947247e95 ("media: videobuf2-vmalloc: get_userptr: buffers are always
> writable"), the pinned pages are always writable.

Actually that patch is only a workaround to temporarily disable
support for read-only pages as they seemed to suffer from some
corruption issues in the retrieved user pages. We expect to support
read-only pages as hardware input after. That said, FOLL_FORCE doesn't
sound like the right thing even in that case, but I don't know the
background behind it being added here in the first place. +Hans
Verkuil +Marek Szyprowski do you happen to remember anything about it?

Best regards,
Tomasz

>
> FOLL_FORCE in this case seems to be a legacy leftover. Let's just remove
> it.
>
> Cc: Tomasz Figa <tfiga@...omium.org>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Mauro Carvalho Chehab <mchehab@...nel.org>
> Signed-off-by: David Hildenbrand <david@...hat.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;
> --
> 2.38.1
>