Re: [Linaro-mm-sig] [PATCH 04/18] dma-fence: prime lockdep annotations

[Jason Gunthorpe]

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