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

[David Hildenbrand]

On 22.11.22 15:07, Hans Verkuil wrote:
> On 11/22/22 13:38, David Hildenbrand wrote:
>> On 22.11.22 13:25, Hans Verkuil wrote:
>>> 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 at redhat.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?
>>
>> Yes. The only problematic corner case I can imagine is if someone has a
>> VMA without write permissions (no PROT_WRITE/VM_WRITE) and wants to pin
>> user space pages as a read buffer. We'd specify now FOLL_WRITE without
>> FOLL_FORCE and GUP would reject that: write access without write
>> permissions is invalid.
> 
> I do not believe this will be an issue.
> 
>>
>> There would be no way around "fixing" this implementation to not specify
>> FOLL_WRITE when only reading from user-space pages. Not sure what the
>> implications are regarding that corruption that was mentioned in
>> 707947247e95.
> 
> Before 707947247e95 the FOLL_WRITE flag was only set for write buffers
> (i.e. video capture, DMA_FROM_DEVICE), not for read buffers (video output,
> DMA_TO_DEVICE). In the video output case there should never be any need
> for drivers to write to the buffer to the best of my knowledge.
> 
> But I have had some complaints about that commit that it causes problems
> in some scenarios, and it has been on my todo list for quite some time now
> to dig deeper into this. I probably should prioritize this for this or
> next week.
> 
>>
>> Having said that, I assume such a scenario is unlikely -- but you might
>> know better how user space usually uses this interface. There would be
>> three options:
>>
>> 1) Leave the FOLL_FORCE hack in for now, which I *really* want to avoid.
>> 2) Remove FOLL_FORCE and see if anybody even notices (this patch) and
>>      leave the implementation as is for now.
>> 3) Remove FOLL_FORCE and fixup the implementation to only specify
>>      FOLL_WRITE if the pages will actually get written to.
>>
>> 3) would most probably ideal, however, I am no expert on that code and
>> can't do it (707947247e95 confuses me). So naive me would go with 2) first.
>>
> 
> Option 3 would be best. And 707947247e95 confuses me as well, and I actually
> wrote it :-) I am wondering whether it was addressed at the right level, but
> as I said, I need to dig a bit deeper into this.

Cool, let me know if I can help!

-- 
Thanks,

David / dhildenb