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

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Date:   Mon, 28 Nov 2022 09:18:47 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Hans Verkuil <hverkuil@...all.nl>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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>,
        Tomasz Figa <tfiga@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Subject: Re: [PATCH mm-unstable v1 16/20] mm/frame-vector: remove FOLL_FORCE
 usage

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!

-- 
Thanks,

David / dhildenb