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

[Ralph Campbell]

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: "JÃrÃme 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.