Re: commit 7ffb791423c7 breaks steam game

[Balbir Singh <balbirs@nvidia.com>]

On 3/22/25 23:23, Bert Karwatzki wrote:
> The problem occurs in this part of ttm_tt_populate(), in the nokaslr case
> the loop is entered and repeatedly run because ttm_dma32_pages allocated exceeds
> the ttm_dma32_pages_limit which leads to lots of calls to ttm_global_swapout().
> 
> if (!strcmp(get_current()->comm, "stellaris"))
> 	printk(KERN_INFO "%s: ttm_pages_allocated=0x%llx ttm_pages_limit=0x%lx ttm_dma32_pages_allocated=0x%llx ttm_dma32_pages_limit=0x%lx\n",
> 			__func__, ttm_pages_allocated.counter, ttm_pages_limit, ttm_dma32_pages_allocated.counter, ttm_dma32_pages_limit);
> while (atomic_long_read(&ttm_pages_allocated) > ttm_pages_limit ||
>        atomic_long_read(&ttm_dma32_pages_allocated) >
>        ttm_dma32_pages_limit) {
> 
> 	if (!strcmp(get_current()->comm, "stellaris"))
> 	printk(KERN_INFO "%s: count=%d ttm_pages_allocated=0x%llx ttm_pages_limit=0x%lx ttm_dma32_pages_allocated=0x%llx ttm_dma32_pages_limit=0x%lx\n",
> 			__func__, count++, ttm_pages_allocated.counter, ttm_pages_limit, ttm_dma32_pages_allocated.counter, ttm_dma32_pages_limit);
> 	ret = ttm_global_swapout(ctx, GFP_KERNEL);
> 	if (ret == 0)
> 		break;
> 	if (ret < 0)
> 		goto error;
> }
> 
> In the case without nokaslr on the number of ttm_dma32_pages_allocated is 0 because
> use_dma32 == false in this case.
> 
> So why is use_dma32 enabled with nokaslr? Some more printk()s give this result:
> 
> The GPUs:
> built-in:
> 08:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Cezanne [Radeon Vega Series / Radeon Vega Mobile Series] (rev c5)
> discrete:
> 03:00.0 Display controller: Advanced Micro Devices, Inc. [AMD/ATI] Navi 23 [Radeon RX 6600/6600 XT/6600M] (rev c3)
> 
> With nokaslr:
> [    1.266517] [    T328] dma_addressing_limited: mask = 0xfffffffffff bus_dma_limit = 0x0 required_mask = 0xfffffffff
> [    1.266519] [    T328] dma_addressing_limited: ops = 0000000000000000 use_dma_iommu(dev) = 0
> [    1.266520] [    T328] dma_direct_all_ram_mapped: returning true
> [    1.266521] [    T328] dma_addressing_limited: returning ret = 0
> [    1.266521] [    T328] amdgpu 0000:03:00.0: amdgpu: amdgpu_ttm_init: calling ttm_device_init() with use_dma32 = 0
> [    1.266525] [    T328] entering ttm_device_init, use_dma32 = 0
> [    1.267115] [    T328] entering ttm_pool_init, use_dma32 = 0
> 
> [    3.965669] [    T328] dma_addressing_limited: mask = 0xfffffffffff bus_dma_limit = 0x0 required_mask = 0x3fffffffffff
> [    3.965671] [    T328] dma_addressing_limited: returning true
> [    3.965672] [    T328] amdgpu 0000:08:00.0: amdgpu: amdgpu_ttm_init: calling ttm_device_init() with use_dma32 = 1
> [    3.965674] [    T328] entering ttm_device_init, use_dma32 = 1
> [    3.965747] [    T328] entering ttm_pool_init, use_dma32 = 1
> 
> Without nokaslr:
> [    1.300907] [    T351] dma_addressing_limited: mask = 0xfffffffffff bus_dma_limit = 0x0 required_mask = 0xfffffffff
> [    1.300909] [    T351] dma_addressing_limited: ops = 0000000000000000 use_dma_iommu(dev) = 0
> [    1.300910] [    T351] dma_direct_all_ram_mapped: returning true
> [    1.300910] [    T351] dma_addressing_limited: returning ret = 0
> [    1.300911] [    T351] amdgpu 0000:03:00.0: amdgpu: amdgpu_ttm_init: calling ttm_device_init() with use_dma32 = 0
> [    1.300915] [    T351] entering ttm_device_init, use_dma32 = 0
> [    1.301210] [    T351] entering ttm_pool_init, use_dma32 = 0
> 
> [    4.000602] [    T351] dma_addressing_limited: mask = 0xfffffffffff bus_dma_limit = 0x0 required_mask = 0xfffffffffff
> [    4.000603] [    T351] dma_addressing_limited: ops = 0000000000000000 use_dma_iommu(dev) = 0
> [    4.000604] [    T351] dma_direct_all_ram_mapped: returning true
> [    4.000605] [    T351] dma_addressing_limited: returning ret = 0
> [    4.000606] [    T351] amdgpu 0000:08:00.0: amdgpu: amdgpu_ttm_init: calling ttm_device_init() with use_dma32 = 0
> [    4.000610] [    T351] entering ttm_device_init, use_dma32 = 0
> [    4.000687] [    T351] entering ttm_pool_init, use_dma32 = 0
> 
> So with nokaslr the reuqired mask for the built-in GPU changes from 0xfffffffffff
> to 0x3fffffffffff which causes dma_addressing_limited to return true which causes
> the ttm_device init to be called with use_dma32 = true.

Thanks, this is really the root cause, from what I understand.

>  It also show that for the discreate GPU nothing changes so the bug does not occur
> there.
> 
> I also was able to work around the bug by calling ttm_device_init() with use_dma32=false
> from amdgpu_ttm_init()  (drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c) but I'm not sure if this
> has unwanted side effects.
> 
> int amdgpu_ttm_init(struct amdgpu_device *adev)
> {
> 	uint64_t gtt_size;
> 	int r;
> 
> 	mutex_init(&adev->mman.gtt_window_lock);
> 
> 	dma_set_max_seg_size(adev->dev, UINT_MAX);
> 	/* No others user of address space so set it to 0 */
> 	dev_info(adev->dev, "%s: calling ttm_device_init() with use_dma32 = 0 ignoring %d\n", __func__, dma_addressing_limited(adev->dev));
> 	r = ttm_device_init(&adev->mman.bdev, &amdgpu_bo_driver, adev->dev,
> 			       adev_to_drm(adev)->anon_inode->i_mapping,
> 			       adev_to_drm(adev)->vma_offset_manager,
> 			       adev->need_swiotlb,
> 			       false /* use_dma32 */);
> 	if (r) {
> 		DRM_ERROR("failed initializing buffer object driver(%d).\n", r);
> 		return r;
> 	}
> 

I think this brings us really close, instead of forcing use_dma32 to false, I wonder if we need something like

uin64_t dma_bits = fls64(dma_get_mask(adev->dev));

to ttm_device_init, pass the last argument (use_dma32) as dma_bits < 32?


Thanks,
Balbir Singh