Re: [PATCH mmotm] mm/thp: refix __split_huge_pmd_locked() for migration PMD

[Yang Shi <shy828301@gmail.com>]

On Wed, Mar 2, 2022 at 5:43 PM Hugh Dickins <hughd@google.com> wrote:
>
> Migration entries do not contribute to a page's reference count: move
> __split_huge_pmd_locked()'s page_ref_add() into pmd_migration's else
> block (along with the page_count() check - a page is quite likely to
> to have reference count frozen to 0 when a migration entry is found).
>
> This will fix a very rare anonymous memory leak, after a split_huge_pmd()
> raced with an anon split_huge_page() or an anon THP migrate_pages(): since
> the wrongly raised refcount stopped the page (perhaps small, perhaps huge,
> depending on when the race hit) from ever being freed.  At first I thought
> there were worse risks, from prematurely unfreezing a frozen page: but now
> think that would only affect page cache pages, which do not come this way
> (except for anonymous pages in swap cache, perhaps).

Thanks for catching this. I agree there may be anon memory leak due to
bumped refcount. But I don't think it could affect page cache page
since that code (bumping refcount) is never called for page cache page
IIUC.

The patch looks good to me. Reviewed-by: Yang Shi <shy828301@gmail.com>

>
> Fixes: ec0abae6dcdf ("mm/thp: fix __split_huge_pmd_locked() for migration PMD")
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
> That's an unfair "Fixes": it did not introduce the problem, but it
> missed this aspect of the problem; and will be a good guide to where this
> refix should go if stable backports are asked for.
>
>  mm/huge_memory.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2039,9 +2039,9 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>                 young = pmd_young(old_pmd);
>                 soft_dirty = pmd_soft_dirty(old_pmd);
>                 uffd_wp = pmd_uffd_wp(old_pmd);
> +               VM_BUG_ON_PAGE(!page_count(page), page);
> +               page_ref_add(page, HPAGE_PMD_NR - 1);
>         }
> -       VM_BUG_ON_PAGE(!page_count(page), page);
> -       page_ref_add(page, HPAGE_PMD_NR - 1);
>
>         /*
>          * Withdraw the table only after we mark the pmd entry invalid.