Re: [Linaro-mm-sig] Re: [PATCH v3] drm/amdgpu: Pin buffers while vmap'ing exported dma-buf objects

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 21.08.25 um 15:13 schrieb Alex Deucher:
> On Thu, Aug 21, 2025 at 4:52 AM Christian König
> <christian.koenig@amd.com> wrote:
>>
>>
>> On 21.08.25 08:40, Thomas Zimmermann wrote:
>>> Current dma-buf vmap semantics require that the mapped buffer remains
>>> in place until the corresponding vunmap has completed.
>>>
>>> For GEM-SHMEM, this used to be guaranteed by a pin operation while creating
>>> an S/G table in import. GEM-SHMEN can now import dma-buf objects without
>>> creating the S/G table, so the pin is missing. Leads to page-fault errors,
>>> such as the one shown below.
>>>
>>> [  102.101726] BUG: unable to handle page fault for address: ffffc90127000000
>>> [...]
>>> [  102.157102] RIP: 0010:udl_compress_hline16+0x219/0x940 [udl]
>>> [...]
>>> [  102.243250] Call Trace:
>>> [  102.245695]  <TASK>
>>> [  102.2477V95]  ? validate_chain+0x24e/0x5e0
>>> [  102.251805]  ? __lock_acquire+0x568/0xae0
>>> [  102.255807]  udl_render_hline+0x165/0x341 [udl]
>>> [  102.260338]  ? __pfx_udl_render_hline+0x10/0x10 [udl]
>>> [  102.265379]  ? local_clock_noinstr+0xb/0x100
>>> [  102.269642]  ? __lock_release.isra.0+0x16c/0x2e0
>>> [  102.274246]  ? mark_held_locks+0x40/0x70
>>> [  102.278177]  udl_primary_plane_helper_atomic_update+0x43e/0x680 [udl]
>>> [  102.284606]  ? __pfx_udl_primary_plane_helper_atomic_update+0x10/0x10 [udl]
>>> [  102.291551]  ? lockdep_hardirqs_on_prepare.part.0+0x92/0x170
>>> [  102.297208]  ? lockdep_hardirqs_on+0x88/0x130
>>> [  102.301554]  ? _raw_spin_unlock_irq+0x24/0x50
>>> [  102.305901]  ? wait_for_completion_timeout+0x2bb/0x3a0
>>> [  102.311028]  ? drm_atomic_helper_calc_timestamping_constants+0x141/0x200
>>> [  102.317714]  ? drm_atomic_helper_commit_planes+0x3b6/0x1030
>>> [  102.323279]  drm_atomic_helper_commit_planes+0x3b6/0x1030
>>> [  102.328664]  drm_atomic_helper_commit_tail+0x41/0xb0
>>> [  102.333622]  commit_tail+0x204/0x330
>>> [...]
>>> [  102.529946] ---[ end trace 0000000000000000 ]---
>>> [  102.651980] RIP: 0010:udl_compress_hline16+0x219/0x940 [udl]
>>>
>>> In this stack strace, udl (based on GEM-SHMEM) imported and vmap'ed a
>>> dma-buf from amdgpu. Amdgpu relocated the buffer, thereby invalidating the
>>> mapping.
>>>
>>> Provide a custom dma-buf vmap method in amdgpu that pins the object before
>>> mapping it's buffer's pages into kernel address space. Do the opposite in
>>> vunmap.
>>>
>>> Note that dma-buf vmap differs from GEM vmap in how it handles relocation.
>>> While dma-buf vmap keeps the buffer in place, GEM vmap requires the caller
>>> to keep the buffer in place. Hence, this fix is in amdgpu's dma-buf code
>>> instead of its GEM code.
>>>
>>> A discussion of various approaches to solving the problem is available
>>> at [1].
>>>
>>> v3:
>>> - try (GTT | VRAM); drop CPU domain (Christian)
>>> v2:
>>> - only use mapable domains (Christian)
>>> - try pinning to domains in preferred order
>>>
>>> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
>>> Fixes: 660cd44659a0 ("drm/shmem-helper: Import dmabuf without mapping its sg_table")
>>> Reported-by: Thomas Zimmermann <tzimmermann@suse.de>
>>> Closes: https://lore.kernel.org/dri-devel/ba1bdfb8-dbf7-4372-bdcb-df7e0511c702@suse.de/
>>> Cc: Shixiong Ou <oushixiong@kylinos.cn>
>>> Cc: Thomas Zimmermann <tzimmermann@suse.de>
>>> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>>> Cc: Maxime Ripard <mripard@kernel.org>
>>> Cc: David Airlie <airlied@gmail.com>
>>> Cc: Simona Vetter <simona@ffwll.ch>
>>> Cc: Sumit Semwal <sumit.semwal@linaro.org>
>>> Cc: "Christian König" <christian.koenig@amd.com>
>>> Cc: dri-devel@lists.freedesktop.org
>>> Cc: linux-media@vger.kernel.org
>>> Cc: linaro-mm-sig@lists.linaro.org
>>> Link: https://lore.kernel.org/dri-devel/9792c6c3-a2b8-4b2b-b5ba-fba19b153e21@suse.de/ # [1]
>> Reviewed-by: Christian König <christian.koenig@amd.com>
> Thomas did you want to take this through drm-misc or do you want me to
> pick this up?

If you haven't send out this week's fixes for amdgpu, you're welcome to 
pick it up. Otherwise I can merge it via drm-misc-fixes next week.

Best regards
Thomas

>
> Thanks,
>
> Alex
>
>>> ---
>>>   drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 34 +++++++++++++++++++--
>>>   1 file changed, 32 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>> index 5743ebb2f1b7..ce27cb5bb05e 100644
>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>>> @@ -285,6 +285,36 @@ static int amdgpu_dma_buf_begin_cpu_access(struct dma_buf *dma_buf,
>>>        return ret;
>>>   }
>>>
>>> +static int amdgpu_dma_buf_vmap(struct dma_buf *dma_buf, struct iosys_map *map)
>>> +{
>>> +     struct drm_gem_object *obj = dma_buf->priv;
>>> +     struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
>>> +     int ret;
>>> +
>>> +     /*
>>> +      * Pin to keep buffer in place while it's vmap'ed. The actual
>>> +      * domain is not that important as long as it's mapable. Using
>>> +      * GTT and VRAM should be compatible with most use cases.
>>> +      */
>>> +     ret = amdgpu_bo_pin(bo, AMDGPU_GEM_DOMAIN_GTT | AMDGPU_GEM_DOMAIN_VRAM);
>>> +     if (ret)
>>> +             return ret;
>>> +     ret = drm_gem_dmabuf_vmap(dma_buf, map);
>>> +     if (ret)
>>> +             amdgpu_bo_unpin(bo);
>>> +
>>> +     return ret;
>>> +}
>>> +
>>> +static void amdgpu_dma_buf_vunmap(struct dma_buf *dma_buf, struct iosys_map *map)
>>> +{
>>> +     struct drm_gem_object *obj = dma_buf->priv;
>>> +     struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
>>> +
>>> +     drm_gem_dmabuf_vunmap(dma_buf, map);
>>> +     amdgpu_bo_unpin(bo);
>>> +}
>>> +
>>>   const struct dma_buf_ops amdgpu_dmabuf_ops = {
>>>        .attach = amdgpu_dma_buf_attach,
>>>        .pin = amdgpu_dma_buf_pin,
>>> @@ -294,8 +324,8 @@ const struct dma_buf_ops amdgpu_dmabuf_ops = {
>>>        .release = drm_gem_dmabuf_release,
>>>        .begin_cpu_access = amdgpu_dma_buf_begin_cpu_access,
>>>        .mmap = drm_gem_dmabuf_mmap,
>>> -     .vmap = drm_gem_dmabuf_vmap,
>>> -     .vunmap = drm_gem_dmabuf_vunmap,
>>> +     .vmap = amdgpu_dma_buf_vmap,
>>> +     .vunmap = amdgpu_dma_buf_vunmap,
>>>   };
>>>
>>>   /**
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-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)