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

Mike Kravetz
2022-05-05T23:53:35Z

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.

> - /* 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.
--
Mike Kravetz