Re: [PATCH v6 01/10] mm: add zone device coherent type memory support

[Alistair Popple <apopple@nvidia.com>]

On Wednesday, 16 February 2022 1:03:57 PM AEDT Jason Gunthorpe wrote:
> On Wed, Feb 16, 2022 at 12:23:44PM +1100, Alistair Popple wrote:
> 
> > Device private and device coherent pages are not marked with pte_devmap and they
> > are backed by a struct page. The only way of inserting them is via migrate_vma.
> > The refcount is decremented in zap_pte_range() on munmap() with special handling
> > for device private pages. Looking at it again though I wonder if there is any
> > special treatment required in zap_pte_range() for device coherent pages given
> > they count as present pages.
> 
> This is what I guessed, but we shouldn't be able to just drop
> pte_devmap on these pages without any other work?? Granted it does
> very little already..

Yes, I agree we need to check this more closely. For device private pages
not having pte_devmap is fine, because they are non-present swap entries so
they always get special handling in the swap entry paths but the same isn't
true for coherent device pages.

> I thought at least gup_fast needed to be touched or did this get
> handled by scanning the page list after the fact?

Right, for gup I think the only special handling required is to prevent
pinning. I had assumed that check_and_migrate_movable_pages() would still get
called for gup_fast but unless I've missed something I don't think it does.
That means gup_fast could still pin movable and coherent pages. Technically
that is ok for coherent pages, but it's undesirable.

 - Alistair

> Jason
>