On 18.08.25 10:41, 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].
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.... 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.... # [1]
drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 36 +++++++++++++++++++-- 1 file changed, 34 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..5b33776eeece 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c @@ -285,6 +285,38 @@ 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* location is not important as long as it's mapable.
Yeah, exactly that won't work here. Most of the locations are not CPU accessible.
You could use AMDGPU_GEM_DOMAIN_GTT, that should most likely work in all cases but isn't necessarily the most optimal solution.
Regards, Christian.
** This code is required for exporting to GEM-SHMEM without S/G table.* Once GEM-SHMEM supports dynamic imports, it should be dropped.*/- ret = amdgpu_bo_pin(bo, AMDGPU_GEM_DOMAIN_MASK);
- 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 +326,8 @@ 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,
}; /**