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

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Date:   Thu, 8 Aug 2019 14:29:34 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Christoph Hellwig <hch@....de>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <amd-gfx@...ts.freedesktop.org>, <dri-devel@...ts.freedesktop.org>,
        <nouveau@...ts.freedesktop.org>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Ben Skeggs <bskeggs@...hat.com>
Subject: Re: [PATCH] nouveau/hmm: map pages after migration


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@...dia.com>
>> Cc: Christoph Hellwig <hch@....de>
>> Cc: Jason Gunthorpe <jgg@...lanox.com>
>> Cc: "Jérôme Glisse" <jglisse@...hat.com>
>> Cc: Ben Skeggs <bskeggs@...hat.com>
>> ---
>>
>> This patch is based on top of Christoph Hellwig's 9 patch series
>> https://lore.kernel.org/linux-mm/20190729234611.GC7171@redhat.com/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.