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

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Message-ID: <6175d780-3307-854c-448a-8e6c7ad0772c@xs4all.nl>
Date:   Tue, 22 Nov 2022 13:25:45 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Tomasz Figa <tfiga@...omium.org>,
        David Hildenbrand <david@...hat.com>,
        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 Tomasz, David,

On 11/8/22 05:45, Tomasz Figa wrote:
> 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?

I tracked the use of 'force' all the way back to the first git commit
(2.6.12-rc1) in the very old video-buf.c. So it is very, very old and the
reason is lost in the mists of time.

I'm not sure if the 'force' argument of get_user_pages() at that time
even meant the same as FOLL_FORCE today. From what I can tell it has just
been faithfully used ever since, but I have my doubt that anyone understands
the reason behind it since it was never explained.

Looking at this old LWN article https://lwn.net/Articles/28548/ suggests
that it might be related to calling get_user_pages for write buffers
(non-zero write argument) where you also want to be able to read from the
buffer. That is certainly something that some drivers need to do post-capture
fixups.

But 'force' was also always set for read buffers, and I don't know if that
was something that was actually needed, or just laziness.

I assume that removing FOLL_FORCE from 'FOLL_FORCE|FOLL_WRITE' will still
allow drivers to read from the buffer?

Regards,

	Hans

> 
> 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
>>