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

[David Hildenbrand]

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.

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.

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.

-- 
Thanks,

David / dhildenb