Re: [PATCH] drm/radeon: Convert get_user_pages() --> pin_user_pages()

From: John Hubbard
Date: Wed May 27 2020 - 15:07:14 EST

On 2020-05-27 01:51, Daniel Vetter wrote:
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":

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.

Aha, thanks Daniel. I withdraw my misinformed ACK, then.

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).

This is what case 3 was *intended* to cover, but it looks like case 3 needs to
be written a little better. I'll attempt that, and Cc you on the actual patch
to -mm. (I think we also need a case 5 for an unrelated scenario, too, so
it's time.)

John Hubbard

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@xxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>


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)
- 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)

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@xxxxxxxxxx>

John Hubbard

- put_page(page);
+ unpin_user_page(page);

Daniel Vetter
Software Engineer, Intel Corporation