On Tue, Nov 22, 2022 at 4:25 AM Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
>
> 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.
Well, not entirely.
For archaeology reasons, I went back to the old BK history, which
exists as a git conversion in
https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/
and there you can actually find it.
Not with a lot of explanations, though - it's commit b7649ef789
("[PATCH] videobuf update"):
This updates the video-buf.c module (helper module for video buffer
management). Some memory management fixes, also some adaptions to the
final v4l2 api.
but it went from
err = get_user_pages(current,current->mm,
- data, dma->nr_pages,
- rw == READ, 0, /* don't force */
+ data & PAGE_MASK, dma->nr_pages,
+ rw == READ, 1, /* force */
dma->pages, NULL);
in that commit.
So it goes back to October 2002.
> 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
The timing roughly matches.
> I assume that removing FOLL_FORCE from 'FOLL_FORCE|FOLL_WRITE' will still
> allow drivers to read from the buffer?
The issue with some of the driver hacks has been that
- they only want to read, and the buffer may be read-only
- they then used FOLL_WRITE despite that, because they want to break
COW (due to the issue that David is now fixing with his series)
- but that means that the VM layer says "nope, you can't write to
this read-only user mapping"
- ... and then they use FOLL_FORCE to say "yes, I can".
iOW, the FOLL_FORCE may be entirely due to an (incorrect, but
historically needed) FOLL_WRITE.
Linus
