Re: [PATCH 2/2] mm: remove device private page support from hmm_range_fault - dri-devel - freedesktop.org experimental mailing list

16 Mar 2020


      On 3/16/20 11:49 AM, Christoph Hellwig wrote:
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On Mon, Mar 16, 2020 at 11:42:19AM -0700, Ralph Campbell wrote:
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On 3/16/20 10:52 AM, Christoph Hellwig wrote:
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No driver has actually used properly wire up and support this feature.
There is various code related to it in nouveau, but as far as I can tell
it never actually got turned on, and the only changes since the initial
commit are global cleanups.
This is not actually true. OpenCL 2.x does support SVM with nouveau and
device private memory via clEnqueueSVMMigrateMem().
Also, Ben Skeggs has accepted a set of patches to map GPU memory after being
migrated and this change would conflict with that.
Can you explain me how we actually invoke this code?
GPU memory is allocated when the device private memory "struct page" is
allocated. See where nouveau_dmem_chunk_alloc() calls nouveau_bo_new().
Then when a page is migrated to the GPU, the GPU memory physical address
is just the offset into the "fake" PFN range allocated by
devm_request_free_mem_region().
I'm looking into allocating GPU memory at the time of migration instead of when
the device private memory struct pages are allocated but that is a future
improvement.
System memory is migrated to GPU memory:
# mesa
clEnqueueSVMMigrateMem()
   svm_migrate_op()
     q.svm_migrate()
       pipe->svm_migrate() // really nvc0_svm_migrate()
         drmCommandWrite() // in libdrm
           drmIoctl()
             ioctl()
               nouveau_drm_ioctl() // nouveau_drm.c
                 drm_ioctl()
                   nouveau_svmm_bind()
                     nouveau_dmem_migrate_vma()
                       migrate_vma_setup()
                       nouveau_dmem_migrate_chunk()
                         nouveau_dmem_migrate_copy_one()
                           // allocate device private struct page
                           dpage = nouveau_dmem_page_alloc_locked()
                             dpage = nouveau_dmem_pages_alloc()
                             // Get GPU VRAM physical address
                             nouveau_dmem_page_addr(dpage)
                             // This does the DMA to GPU memory
                             drm->dmem->migrate.copy_func()
                       migrate_vma_pages()
                       migrate_vma_finalize()
Without my recent patch set, there is no GPU page table entry created for
this migrated memory so there will be a GPU fault which is handled in a
worker thread:
nouveau_svm_fault()
   // examine fault buffer entries and compute range of pages
   nouveau_range_fault()
     // This will fill in the pfns array with a device private entry PFN
     hmm_range_fault()
     // This sees the range->flags[HMM_PFN_DEVICE_PRIVATE] flag
     // and converts the HMM PFN to a GPU physical address
     nouveau_dmem_convert_pfn()
     // This sets up the GPU page tables
     nvif_object_ioctl()
...
For that we'd need HMM_PFN_DEVICE_PRIVATE NVIF_VMM_PFNMAP_V0_VRAM
set in ->pfns before calling hmm_range_fault, which isn't happening.
It is set by hmm_range_fault() via the range->flags[HMM_PFN_DEVICE_PRIVATE] entry
when hmm_range_fault() sees a device private struct page. The call to
nouveau_dmem_convert_pfn() is just replacing the "fake" PFN with the real PFN
but not clearing/changing the read/write or VRAM/system memory PTE bits.