Re: [PATCH] nouveau/hmm: map pages after migration

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On 8/8/19 12:07 AM, Christoph Hellwig wrote:
On Wed, Aug 07, 2019 at 08:02:14AM -0700, Ralph Campbell wrote:
When memory is migrated to the GPU it is likely to be accessed by GPU
code soon afterwards. Instead of waiting for a GPU fault, map the
migrated memory into the GPU page tables with the same access permissions
as the source CPU page table entries. This preserves copy on write
semantics.

Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
Cc: Ben Skeggs <bskeggs@xxxxxxxxxx>
---

This patch is based on top of Christoph Hellwig's 9 patch series
https://lore.kernel.org/linux-mm/20190729234611.GC7171@xxxxxxxxxx/T/#u
"turn the hmm migrate_vma upside down" but without patch 9
"mm: remove the unused MIGRATE_PFN_WRITE" and adds a use for the flag.

This looks useful.  I've already dropped that patch for the pending
resend.

Thanks.


  static unsigned long nouveau_dmem_migrate_copy_one(struct nouveau_drm *drm,
-		struct vm_area_struct *vma, unsigned long addr,
-		unsigned long src, dma_addr_t *dma_addr)
+		struct vm_area_struct *vma, unsigned long src,
+		dma_addr_t *dma_addr, u64 *pfn)

I'll pick up the removal of the not needed addr argument for the patch
introducing nouveau_dmem_migrate_copy_one, thanks,

  static void nouveau_dmem_migrate_chunk(struct migrate_vma *args,
-		struct nouveau_drm *drm, dma_addr_t *dma_addrs)
+		struct nouveau_drm *drm, dma_addr_t *dma_addrs, u64 *pfns)
  {
  	struct nouveau_fence *fence;
  	unsigned long addr = args->start, nr_dma = 0, i;
for (i = 0; addr < args->end; i++) {
  		args->dst[i] = nouveau_dmem_migrate_copy_one(drm, args->vma,
-				addr, args->src[i], &dma_addrs[nr_dma]);
+				args->src[i], &dma_addrs[nr_dma], &pfns[i]);

Nit: I find the &pfns[i] way to pass the argument a little weird to read.
Why not "pfns + i"?

OK, will do in v2.
Should I convert to "dma_addrs + nr_dma" too?

+u64 *
+nouveau_pfns_alloc(unsigned long npages)
+{
+	struct nouveau_pfnmap_args *args;
+
+	args = kzalloc(sizeof(*args) + npages * sizeof(args->p.phys[0]),

Can we use struct_size here?

Yes, good suggestion.


+	int ret;
+
+	if (!svm)
+		return;
+
+	mutex_lock(&svm->mutex);
+	svmm = nouveau_find_svmm(svm, mm);
+	if (!svmm) {
+		mutex_unlock(&svm->mutex);
+		return;
+	}
+	mutex_unlock(&svm->mutex);

Given that nouveau_find_svmm doesn't take any kind of reference, what
gurantees svmm doesn't go away after dropping the lock?

I asked Ben and Jerome about this too.
I'm still looking into it.


@@ -44,5 +49,19 @@ static inline int nouveau_svmm_bind(struct drm_device *device, void *p,
  {
  	return -ENOSYS;
  }
+
+u64 *nouveau_pfns_alloc(unsigned long npages)
+{
+	return NULL;
+}
+
+void nouveau_pfns_free(u64 *pfns)
+{
+}
+
+void nouveau_pfns_map(struct nouveau_drm *drm, struct mm_struct *mm,
+		      unsigned long addr, u64 *pfns, unsigned long npages)
+{
+}
  #endif /* IS_ENABLED(CONFIG_DRM_NOUVEAU_SVM) */

nouveau_dmem.c and nouveau_svm.c are both built conditional on
CONFIG_DRM_NOUVEAU_SVM, so there is no need for stubs here.


Good point. I'll remove them in v2.
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