Re: [RESEND PATCH 2/3] nouveau: fix mixed normal and device private page migration

[Ralph Campbell <rcampbell@nvidia.com>]

On 6/25/20 10:31 AM, Jason Gunthorpe wrote:
> On Thu, Jun 25, 2020 at 10:25:38AM -0700, Ralph Campbell wrote:
>> Making sure to include linux-mm and Bharata B Rao for IBM's
>> use of migrate_vma*().
>>
>> On 6/24/20 11:10 AM, Ralph Campbell wrote:
>>>
>>> On 6/24/20 12:23 AM, Christoph Hellwig wrote:
>>>> On Mon, Jun 22, 2020 at 04:38:53PM -0700, Ralph Campbell wrote:
>>>>> The OpenCL function clEnqueueSVMMigrateMem(), without any flags, will
>>>>> migrate memory in the given address range to device private memory. The
>>>>> source pages might already have been migrated to device private memory.
>>>>> In that case, the source struct page is not checked to see if it is
>>>>> a device private page and incorrectly computes the GPU's physical
>>>>> address of local memory leading to data corruption.
>>>>> Fix this by checking the source struct page and computing the correct
>>>>> physical address.
>>>>
>>>> I'm really worried about all this delicate code to fix the mixed
>>>> ranges.  Can't we make it clear at the migrate_vma_* level if we want
>>>> to migrate from or two device private memory, and then skip all the work
>>>> for regions of memory that already are in the right place?  This might be
>>>> a little more work initially, but I think it leads to a much better
>>>> API.
>>>>
>>>
>>> The current code does encode the direction with src_owner != NULL meaning
>>> device private to system memory and src_owner == NULL meaning system
>>> memory to device private memory. This patch would obviously defeat that
>>> so perhaps a flag could be added to the struct migrate_vma to indicate the
>>> direction but I'm unclear how that makes things less delicate.
>>> Can you expand on what you are worried about?
>>>
>>> The issue with invalidations might be better addressed by letting the device
>>> driver handle device private page TLB invalidations when migrating to
>>> system memory and changing migrate_vma_setup() to only invalidate CPU
>>> TLB entries for normal pages being migrated to device private memory.
>>> If a page isn't migrating, it seems inefficient to invalidate those TLB
>>> entries.
>>>
>>> Any other suggestions?
>>
>> After a night's sleep, I think this might work. What do others think?
>>
>> 1) Add a new MMU_NOTIFY_MIGRATE enum to mmu_notifier_event.
>>
>> 2) Change migrate_vma_collect() to use the new MMU_NOTIFY_MIGRATE event type.
>>
>> 3) Modify nouveau_svmm_invalidate_range_start() to simply return (no invalidations)
>> for MMU_NOTIFY_MIGRATE mmu notifier callbacks.
> 
> Isn't it a bit of an assumption that migrate_vma_collect() is only
> used by nouveau itself?
> 
> What if some other devices' device_private pages are being migrated?
> 
> Jason
> 

Good point. The driver needs a way of knowing the callback is due its call
to migrate_vma_setup() and not some other migration invalidation.
How about adding a void pointer to struct mmu_notifier_range
which migrate_vma_collect() can set to src_owner. If the event is
MMU_NOTIFY_MIGRATE and the src_owner matches the void pointer, then the
callback should be the one the driver initiated.