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SubjectRe: [PATCH] drm/radeon: Convert get_user_pages() --> pin_user_pages()
On Wed, May 27, 2020 at 10:48:52AM +0200, Daniel Vetter wrote:
> On Tue, May 26, 2020 at 03:57:45PM -0700, John Hubbard wrote:
> > On 2020-05-26 14:00, Souptick Joarder wrote:
> > > This code was using get_user_pages(), in a "Case 2" scenario
> > > (DMA/RDMA), using the categorization from [1]. That means that it's
> > > time to convert the get_user_pages() + release_pages() calls to
> > > pin_user_pages() + unpin_user_pages() calls.
> > >
> > > There is some helpful background in [2]: basically, this is a small
> > > part of fixing a long-standing disconnect between pinning pages, and
> > > file systems' use of those pages.
> > >
> > > [1] Documentation/core-api/pin_user_pages.rst
> > >
> > > [2] "Explicit pinning of user-space pages":
> > > https://lwn.net/Articles/807108/
>
> I don't think this is a case 2 here, nor is it any of the others. Feels
> like not covered at all by the doc.
>
> radeon has a mmu notifier (might be a bit broken, but hey whatever there's
> other drivers which have the same concept, but less broken). So when you
> do an munmap, radeon will release the page refcount.

I forgot to add: It's also not case 3, since there's no hw page fault
support. It's all faked in software, and explicitly synchronizes against
pending io (or preempts it, that depends a bit upon the jobs running).

> Which case it that?
>
> Note that currently only amdgpu doesn't work like that for gpu dma
> directly to userspace ranges, it uses hmm and afaiui doens't hold a full
> page pin refcount.
>
> Cheers, Daniel
>
>
> > >
> > > Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
> > > Cc: John Hubbard <jhubbard@nvidia.com>
> > >
> > > Hi,
> > >
> > > I'm compile tested this, but unable to run-time test, so any testing
> > > help is much appriciated.
> > > ---
> > > drivers/gpu/drm/radeon/radeon_ttm.c | 6 +++---
> > > 1 file changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/radeon/radeon_ttm.c b/drivers/gpu/drm/radeon/radeon_ttm.c
> > > index 5d50c9e..e927de2 100644
> > > --- a/drivers/gpu/drm/radeon/radeon_ttm.c
> > > +++ b/drivers/gpu/drm/radeon/radeon_ttm.c
> > > @@ -506,7 +506,7 @@ static int radeon_ttm_tt_pin_userptr(struct ttm_tt *ttm)
> > > uint64_t userptr = gtt->userptr + pinned * PAGE_SIZE;
> > > struct page **pages = ttm->pages + pinned;
> > > - r = get_user_pages(userptr, num_pages, write ? FOLL_WRITE : 0,
> > > + r = pin_user_pages(userptr, num_pages, write ? FOLL_WRITE : 0,
> > > pages, NULL);
> > > if (r < 0)
> > > goto release_pages;
> > > @@ -535,7 +535,7 @@ static int radeon_ttm_tt_pin_userptr(struct ttm_tt *ttm)
> > > kfree(ttm->sg);
> > > release_pages:
> > > - release_pages(ttm->pages, pinned);
> > > + unpin_user_pages(ttm->pages, pinned);
> > > return r;
> > > }
> > > @@ -562,7 +562,7 @@ static void radeon_ttm_tt_unpin_userptr(struct ttm_tt *ttm)
> > > set_page_dirty(page);
> >
> >
> > Maybe we also need a preceding patch, to fix the above? It should be
> > set_page_dirty_lock(), rather than set_page_dirty(), unless I'm overlooking
> > something (which is very possible!).
> >
> > Either way, from a tunnel vision perspective of changing gup to pup, this
> > looks good to me, so
> >
> > Acked-by: John Hubbard <jhubbard@nvidia.com>
> >
> >
> > thanks,
> > --
> > John Hubbard
> > NVIDIA
> >
> > > mark_page_accessed(page);
> > > - put_page(page);
> > > + unpin_user_page(page);
> > > }
> > > sg_free_table(ttm->sg);
> > >
> >
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

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

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