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

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Message-ID: <0a52ed02-cf9d-eb67-8d68-e3d9dbe4e7b2@xs4all.nl>
Date:   Mon, 28 Nov 2022 09:26:04 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     David Hildenbrand <david@...hat.com>, 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 28/11/2022 09:18, David Hildenbrand 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!

Andrew, I've resent my patch, this time with you CCed as well.

Regards,

	Hans

> 
> Thanks!
>