On Fri, Jun 19, 2020 at 04:10:11PM -0400, Jerome Glisse wrote:
> Maybe we can audit how user ptr buffer are use today and see if
> we can define a usage pattern that would allow to cut corner in
> kernel. For instance we could use mmu notifier just to block CPU
> pte update while we do GUP and thus never wait on dma fence.
The DMA fence is the main problem, if you can think of a way to avoid
it then it would be great!
> Then GPU driver just keep the GUP pin around until they are done
> with the page. They can also use the mmu notifier to keep a flag
> so that the driver know if it needs to redo a GUP ie:
>
> The notifier path:
> GPU_mmu_notifier_start_callback(range)
> gpu_lock_cpu_pagetable(range)
> for_each_bo_in(bo, range) {
> bo->need_gup = true;
> }
> gpu_unlock_cpu_pagetable(range)
So some kind of invalidation tracking? But this doesn't solve COW and
Fork problem?
> > It is kind of unrelated to HMM, it just shouldn't be using mmu
> > notifiers to replace page pinning..
>
> Well my POV is that if you abide by rules HMM defined then you do
> not need to pin pages. The rule is asynchronous device page table
> update.
I think one of the hmm rules is to not block notifiers for a long
time, which these scheme seem to violate already.
Pinning for a long time is less bad than blocing notifiers for a long
time, IMHO
Jason
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