Re: [PATCH v2 1/5] mm: Non-pmd-mappable, large folios for folio_add_new_anon_rmap()

[Yu Zhao]

On Mon, Jul 3, 2023 at 7:53 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> In preparation for FLEXIBLE_THP support, improve
> folio_add_new_anon_rmap() to allow a non-pmd-mappable, large folio to be
> passed to it. In this case, all contained pages are accounted using the
> "small" pages scheme.

Nit: In this case, all *subpages*  are accounted using the *order-0
folio* (or base page) scheme.

> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>

Reviewed-by: Yu Zhao <yuzhao@xxxxxxxxxx>

>  mm/rmap.c | 26 +++++++++++++++++++-------
>  1 file changed, 19 insertions(+), 7 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 1d8369549424..82ef5ba363d1 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1278,31 +1278,43 @@ void page_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
>   * This means the inc-and-test can be bypassed.
>   * The folio does not have to be locked.
>   *
> - * If the folio is large, it is accounted as a THP.  As the folio
> + * If the folio is pmd-mappable, it is accounted as a THP.  As the folio
>   * is new, it's assumed to be mapped exclusively by a single process.
>   */
>  void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
>                 unsigned long address)
>  {
> -       int nr;
> +       int nr = folio_nr_pages(folio);
> +       int i;
> +       struct page *page;
>
> -       VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
> +       VM_BUG_ON_VMA(address < vma->vm_start ||
> +                       address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
>         __folio_set_swapbacked(folio);
>
> -       if (likely(!folio_test_pmd_mappable(folio))) {
> +       if (!folio_test_large(folio)) {
>                 /* increment count (starts at -1) */
>                 atomic_set(&folio->_mapcount, 0);
> -               nr = 1;
> +               __page_set_anon_rmap(folio, &folio->page, vma, address, 1);
> +       } else if (!folio_test_pmd_mappable(folio)) {
> +               /* increment count (starts at 0) */
> +               atomic_set(&folio->_nr_pages_mapped, nr);
> +
> +               page = &folio->page;
> +               for (i = 0; i < nr; i++, page++, address += PAGE_SIZE) {
> +                       /* increment count (starts at -1) */
> +                       atomic_set(&page->_mapcount, 0);
> +                       __page_set_anon_rmap(folio, page, vma, address, 1);
> +               }

Nit: use folio_page(), e.g.,

  } else if (!folio_test_pmd_mappable(folio)) {
    int i;

    for (i = 0; i < nr; i++) {
      struct page *page = folio_page(folio, i);

      /* increment count (starts at -1) */
      atomic_set(&page->_mapcount, 0);
      __page_set_anon_rmap(folio, page, vma, address + PAGE_SIZE * i, 1);
    }
    /* increment count (starts at 0) */
    atomic_set(&folio->_nr_pages_mapped, nr);
  } else {

>         } else {
>                 /* increment count (starts at -1) */
>                 atomic_set(&folio->_entire_mapcount, 0);
>                 atomic_set(&folio->_nr_pages_mapped, COMPOUND_MAPPED);
> -               nr = folio_nr_pages(folio);
>                 __lruvec_stat_mod_folio(folio, NR_ANON_THPS, nr);
> +               __page_set_anon_rmap(folio, &folio->page, vma, address, 1);
>         }
>
>         __lruvec_stat_mod_folio(folio, NR_ANON_MAPPED, nr);
> -       __page_set_anon_rmap(folio, &folio->page, vma, address, 1);
>  }