Re: [PATCH 2/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when migration

[Baolin Wang]

On 5/6/2022 7:53 AM, Mike Kravetz wrote:
> On 4/29/22 01:14, Baolin Wang wrote:
> > On some architectures (like ARM64), it can support CONT-PTE/PMD size
> > hugetlb, which means it can support not only PMD/PUD size hugetlb:
> > 2M and 1G, but also CONT-PTE/PMD size: 64K and 32M if a 4K page
> > size specified.
> <snip>
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 6fdd198..7cf2408 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1924,13 +1924,15 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
> >   					break;
> >   				}
> >   			}
> > +
> > +			/* Nuke the hugetlb page table entry */
> > +			pteval = huge_ptep_clear_flush(vma, address, pvmw.pte);
> >   		} else {
> >   			flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
> > +			/* Nuke the page table entry. */
> > +			pteval = ptep_clear_flush(vma, address, pvmw.pte);
> >   		}
> >   
>
> On arm64 with CONT-PTE/PMD the returned pteval will have dirty or young set
> if ANY of the PTE/PMDs had dirty or young set.

Right.

> > -		/* Nuke the page table entry. */
> > -		pteval = ptep_clear_flush(vma, address, pvmw.pte);
> > -
> >   		/* Set the dirty flag on the folio now the pte is gone. */
> >   		if (pte_dirty(pteval))
> >   			folio_mark_dirty(folio);
> > @@ -2015,7 +2017,10 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
> >   			pte_t swp_pte;
> >   
> >   			if (arch_unmap_one(mm, vma, address, pteval) < 0) {
> > -				set_pte_at(mm, address, pvmw.pte, pteval);
> > +				if (folio_test_hugetlb(folio))
> > +					set_huge_pte_at(mm, address, pvmw.pte, pteval);
>
> And, we will use that pteval for ALL the PTE/PMDs here.  So, we would set
> the dirty or young bit in ALL PTE/PMDs.
>
> Could that cause any issues?  May be more of a question for the arm64 people.

I don't think this will cause any issues. Since the hugetlb can not be 
split, and we should not lose the the dirty or young state if any 
subpages were set. Meanwhile we already did like this in hugetlb.c:

pte = huge_ptep_get_and_clear(mm, address, ptep);
tlb_remove_huge_tlb_entry(h, tlb, ptep, address);
if (huge_pte_dirty(pte))
	set_page_dirty(page);