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

Daniel Vetter
2020-10-07T21:51:03Z

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