Re: [PATCH RFC 16/19] mm/frame-vector: remove FOLL_FORCE usage

[Hans Verkuil]

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@xxxxxxxxxx> 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@xxxxxxxxxxxx>
>> Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
>> Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
>> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>> ---
>>  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
>>