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Subject: Re: [PATCH] nouveau/hmm: map pages after migration
To:     Christoph Hellwig <hch@lst.de>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        <amd-gfx@lists.freedesktop.org>, <dri-devel@lists.freedesktop.org>,
        <nouveau@lists.freedesktop.org>,
        Jason Gunthorpe <jgg@mellanox.com>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Ben Skeggs <bskeggs@redhat.com>
References: <20190807150214.3629-1-rcampbell@nvidia.com>
 <20190808070701.GC29382@lst.de>
From:   Ralph Campbell <rcampbell@nvidia.com>
Message-ID: <0b96a8d8-86b5-3ce0-db95-669963c1f8a7@nvidia.com>
Date:   Thu, 8 Aug 2019 14:29:34 -0700
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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 permission=
s
>> as the source CPU page table entries. This preserves copy on write
>> semantics.
>>
>> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
>> Cc: Christoph Hellwig <hch@lst.de>
>> Cc: Jason Gunthorpe <jgg@mellanox.com>
>> Cc: "J=C3=A9r=C3=B4me Glisse" <jglisse@redhat.com>
>> Cc: Ben Skeggs <bskeggs@redhat.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.
>=20
> This looks useful.  I've already dropped that patch for the pending
> resend.

Thanks.

>=20
>>   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)
>=20
> I'll pick up the removal of the not needed addr argument for the patch
> introducing nouveau_dmem_migrate_copy_one, thanks,
>=20
>>   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 =3D args->start, nr_dma =3D 0, i;
>>  =20
>>   	for (i =3D 0; addr < args->end; i++) {
>>   		args->dst[i] =3D 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]);
>=20
> 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 =3D kzalloc(sizeof(*args) + npages * sizeof(args->p.phys[0]),
>=20
> Can we use struct_size here?

Yes, good suggestion.

>=20
>> +	int ret;
>> +
>> +	if (!svm)
>> +		return;
>> +
>> +	mutex_lock(&svm->mutex);
>> +	svmm =3D nouveau_find_svmm(svm, mm);
>> +	if (!svmm) {
>> +		mutex_unlock(&svm->mutex);
>> +		return;
>> +	}
>> +	mutex_unlock(&svm->mutex);
>=20
> 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.

>=20
>> @@ -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) */
>=20
> nouveau_dmem.c and nouveau_svm.c are both built conditional on
> CONFIG_DRM_NOUVEAU_SVM, so there is no need for stubs here.
>=20

Good point. I'll remove them in v2.