linux-kernel - Re: [PATCH mm-unstable v1 16/20] mm/frame-vector: remove FOLL

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Date:   Mon, 28 Nov 2022 17:57:51 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Hans Verkuil <hverkuil@...all.nl>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        linux-alpha@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-ia64@...r.kernel.org, linux-mips@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org,
        linux-um@...ts.infradead.org, etnaviv@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-samsung-soc@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-media@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-perf-users@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Linus Torvalds <torvalds@...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>,
        Shuah Khan <shuah@...nel.org>,
        Lucas Stach <l.stach@...gutronix.de>,
        David Airlie <airlied@...il.com>,
        Oded Gabbay <ogabbay@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christoph Hellwig <hch@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Subject: Re: [PATCH mm-unstable v1 16/20] mm/frame-vector: remove FOLL_FORCE usage

On Mon, Nov 28, 2022 at 5:19 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 28.11.22 09:17, Hans Verkuil wrote:
> > Hi David,
> >
> > On 27/11/2022 11:35, David Hildenbrand wrote:
> >> On 16.11.22 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.
> >>>
> >>> 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@...all.nl>
> >>> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> >>> 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;
> >>
> >>
> >> Hi Andrew,
> >>
> >> see the discussion at [1] regarding a conflict and how to proceed with
> >> upstreaming. The conflict would be easy to resolve, however, also
> >> the patch description doesn't make sense anymore with [1].
> >
> > Might it be easier and less confusing if you post a v2 of this series
> > with my patch first? That way it is clear that 1) my patch has to come
> > first, and 2) that it is part of a single series and should be merged
> > by the mm subsystem.
> >
> > Less chances of things going wrong that way.
> >
> > Just mention in the v2 cover letter that the first patch was added to
> > make it easy to backport that fix without being hampered by merge
> > conflicts if it was added after your frame_vector.c patch.
>
> Yes, that's the way I would naturally do, it, however, Andrew prefers
> delta updates for minor changes.
>
> @Andrew, whatever you prefer!
>
> Thanks!
>

However you folks proceed with taking this patch, feel free to add my
Acked-by. Thanks!

Best regards,
Tomasz

> --
> Thanks,
>
> David / dhildenb
>