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

[Ralph Campbell]

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.