Nouveau - Mar 2021 - [PATCH v7 5/8] mm: Device exclusive memory access

On Wednesday, 31 March 2021 6:32:34 AM AEDT Jason Gunthorpe
wrote:
> On Fri, Mar 26, 2021 at 11:08:02AM +1100, Alistair Popple wrote:
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 3a5705cfc891..33d11527ef77 100644
> > +++ b/mm/memory.c
> > @@ -781,6 +781,27 @@ copy_nonpresent_pte(struct mm_struct *dst_mm,
struct
mm_struct *src_mm,
> >  				pte = pte_swp_mkuffd_wp(pte);
> >  			set_pte_at(src_mm, addr, src_pte, pte);
> >  		}
> > +	} else if (is_device_exclusive_entry(entry)) {
> > +		page = pfn_swap_entry_to_page(entry);
> > +
> > +		get_page(page);
> > +		rss[mm_counter(page)]++;
> > +
> > +		if (is_writable_device_exclusive_entry(entry) &&
> > +		    is_cow_mapping(vm_flags)) {
> > +			/*
> > +			 * COW mappings require pages in both
> > +			 * parent and child to be set to read.
> > +			 */
> > +			entry = make_readable_device_exclusive_entry(
> > +							swp_offset(entry));
> > +			pte = swp_entry_to_pte(entry);
> > +			if (pte_swp_soft_dirty(*src_pte))
> > +				pte = pte_swp_mksoft_dirty(pte);
> > +			if (pte_swp_uffd_wp(*src_pte))
> > +				pte = pte_swp_mkuffd_wp(pte);
> > +			set_pte_at(src_mm, addr, src_pte, pte);
> > +		}
> 
> This needs to have the same logic as we now have in
> copy_present_page(). The page *is* present and we can't copy the PTE
> value hidden in a swap entry if we can't copy the PTE normally.
You're saying we need to use copy_present_page() to make sure the split goes
the right way for pinned pages? I guess that makes sense as the split could go 
either way at the moment but I should add a check to make sure this isn't
used
with pinned pages anyway.

 - Alistair
> The code should be shared because nobody is going to remember about
> this corner case.
> 
> Jason
>

On Wed, Mar 31, 2021 at 11:59:28PM +1100, Alistair Popple wrote:
> I guess that makes sense as the split could go either way at the
> moment but I should add a check to make sure this isn't used with
> pinned pages anyway.
Is it possible to have a pinned page under one of these things? If I
pin it before you migrate it then it remains pinned but hidden under
the swap entry?

So the special logic is needed and the pinned page has to be copied
and written as a normal pte, not dropped as a migration entry

Jason