Re: [PATCH 01/13] drm/exynos: Stop using frame_vector helpers

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Wed, Oct 7, 2020 at 11:37 PM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 10/7/20 2:32 PM, Daniel Vetter wrote:
> > On Wed, Oct 7, 2020 at 10:33 PM John Hubbard <jhubbard@nvidia.com> wrote:
> >>
> >> On 10/7/20 9:44 AM, Daniel Vetter wrote:
> ...
> >>> @@ -398,15 +399,11 @@ static void g2d_userptr_put_dma_addr(struct g2d_data *g2d,
> >>>        dma_unmap_sgtable(to_dma_dev(g2d->drm_dev), g2d_userptr->sgt,
> >>>                          DMA_BIDIRECTIONAL, 0);
> >>>
> >>> -     pages = frame_vector_pages(g2d_userptr->vec);
> >>> -     if (!IS_ERR(pages)) {
> >>> -             int i;
> >>> +     for (i = 0; i < g2d_userptr->npages; i++)
> >>> +             set_page_dirty_lock(g2d_userptr->pages[i]);
> >>>
> >>> -             for (i = 0; i < frame_vector_count(g2d_userptr->vec); i++)
> >>> -                     set_page_dirty_lock(pages[i]);
> >>> -     }
> >>> -     put_vaddr_frames(g2d_userptr->vec);
> >>> -     frame_vector_destroy(g2d_userptr->vec);
> >>> +     unpin_user_pages(g2d_userptr->pages, g2d_userptr->npages);
> >>> +     kvfree(g2d_userptr->pages);
> >>
> >> You can avoid writing your own loop, and just simplify the whole thing down to
> >> two lines:
> >>
> >>          unpin_user_pages_dirty_lock(g2d_userptr->pages, g2d_userptr->npages,
> >>                                      true);
> >>          kvfree(g2d_userptr->pages);
> >
> > Oh nice, this is neat. I'll also roll it out in the habanalabs patch,
> > that has the same thing. Well almost, it only uses set_page_dirty, not
> > the _lock variant. But I have no idea whether that matters or not?
>
>
> It matters. And invariably, call sites that use set_page_dirty() instead
> of set_page_dirty_lock() were already wrong.  Which is why I never had to
> provide anything like "unpin_user_pages_dirty (not locked)".
>
> Although in habanalabs case, I just reviewed patch 3 and I think they *were*
> correctly using set_page_dirty_lock()...

Yeah I mixed that up with some other code I read, habanalabs is using
_lock. I have seen a pile of gup/pup code though that only uses
set_page_dirty. And looking around I did not really parse the comment
above set_page_dirty(). I guess just using the _lock variant shouldn't
hurt too much. I've found a comment though from the infiniband umem
notifier that it's sometimes called with the page locked, and
sometimes not, so life is complicated there. But how it avoids races I
didn't understand.
-Daniel


--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch