Re: [PATCH 1/3] mm,thp,rmap: subpages_mapcount of PTE-mapped subpages

[Yu Zhao <yuzhao@google.com>]

On Fri, Nov 18, 2022 at 2:12 AM Hugh Dickins <hughd@google.com> wrote:

...

> @@ -1308,31 +1285,29 @@ void page_add_anon_rmap(struct page *page,
>
>         if (unlikely(PageKsm(page)))
>                 lock_page_memcg(page);
> -       else
> -               VM_BUG_ON_PAGE(!PageLocked(page), page);
>
> -       if (likely(!PageCompound(page))) {
> +       if (likely(!compound /* page is mapped by PTE */)) {
>                 first = atomic_inc_and_test(&page->_mapcount);
>                 nr = first;
> +               if (first && PageCompound(page)) {
> +                       struct page *head = compound_head(page);
> +
> +                       lock_compound_mapcounts(head, &mapcounts);
> +                       mapcounts.subpages_mapcount++;
> +                       nr = !mapcounts.compound_mapcount;
> +                       unlock_compound_mapcounts(head, &mapcounts);
> +               }
> +       } else if (PageTransHuge(page)) {
> +               /* That test is redundant: it's for safety or to optimize out */
>
> -       } else if (compound && PageTransHuge(page)) {
>                 lock_compound_mapcounts(page, &mapcounts);
>                 first = !mapcounts.compound_mapcount;
>                 mapcounts.compound_mapcount++;
>                 if (first) {
> -                       nr = nr_pmdmapped = thp_nr_pages(page);
> -                       if (mapcounts.subpages_mapcount)
> -                               nr = nr_subpages_unmapped(page, nr_pmdmapped);
> +                       nr_pmdmapped = thp_nr_pages(page);
> +                       nr = nr_pmdmapped - mapcounts.subpages_mapcount;
>                 }
>                 unlock_compound_mapcounts(page, &mapcounts);
> -       } else {
> -               struct page *head = compound_head(page);
> -
> -               lock_compound_mapcounts(head, &mapcounts);
> -               mapcounts.subpages_mapcount++;
> -               first = subpage_mapcount_inc(page);
> -               nr = first && !mapcounts.compound_mapcount;
> -               unlock_compound_mapcounts(head, &mapcounts);
>         }
>
>         VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page);

Hi Hugh, I got the following warning from the removed "else" branch.
Is it legit? Thanks.

mm/rmap.c:1236:13: warning: variable 'first' is used uninitialized
whenever 'if' condition is false [-Wsometimes-uninitialized]
        } else if (PageTransHuge(page)) {
                   ^~~~~~~~~~~~~~~~~~~
mm/rmap.c:1248:18: note: uninitialized use occurs here
        VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page);
                        ^~~~~