Re: [PATCH v2] zsmalloc: Fix races between modifications of fullness and isolated

[Nhat Pham <nphamcs@gmail.com>]

On Wed, Jul 26, 2023 at 11:30 PM Andrew Yang <andrew.yang@mediatek.com> wrote:
>
> Since fullness and isolated share the same unsigned int,
> modifications of them should be protected by the same lock.
>
> Signed-off-by: Andrew Yang <andrew.yang@mediatek.com>
> Fixes: c4549b871102 ("zsmalloc: remove zspage isolation for migration")
> ---
> v2: Moving comment too
> ---
>  mm/zsmalloc.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 32f5bc4074df..b58f957429f0 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1777,6 +1777,7 @@ static void replace_sub_page(struct size_class *class, struct zspage *zspage,
>
>  static bool zs_page_isolate(struct page *page, isolate_mode_t mode)
>  {
> +       struct zs_pool *pool;
>         struct zspage *zspage;
>
>         /*
> @@ -1786,9 +1787,10 @@ static bool zs_page_isolate(struct page *page, isolate_mode_t mode)
>         VM_BUG_ON_PAGE(PageIsolated(page), page);
>
>         zspage = get_zspage(page);
> -       migrate_write_lock(zspage);
> +       pool = zspage->pool;
> +       spin_lock(&pool->lock);
>         inc_zspage_isolation(zspage);
> -       migrate_write_unlock(zspage);
> +       spin_unlock(&pool->lock);
>
>         return true;
>  }
> @@ -1854,12 +1856,12 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
>         kunmap_atomic(s_addr);
>
>         replace_sub_page(class, zspage, newpage, page);
> +       dec_zspage_isolation(zspage);
>         /*
>          * Since we complete the data copy and set up new zspage structure,
>          * it's okay to release the pool's lock.
>          */
>         spin_unlock(&pool->lock);
> -       dec_zspage_isolation(zspage);
>         migrate_write_unlock(zspage);
>
>         get_page(newpage);
> @@ -1876,14 +1878,16 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
>
>  static void zs_page_putback(struct page *page)
>  {
> +       struct zs_pool *pool;
>         struct zspage *zspage;
>
>         VM_BUG_ON_PAGE(!PageIsolated(page), page);
>
>         zspage = get_zspage(page);
> -       migrate_write_lock(zspage);
> +       pool = zspage->pool;
> +       spin_lock(&pool->lock);
>         dec_zspage_isolation(zspage);
> -       migrate_write_unlock(zspage);
> +       spin_unlock(&pool->lock);
>  }
>
>  static const struct movable_operations zsmalloc_mops = {
> --
> 2.18.0
>
>

I think this fixes an issue of mine that has been bugging me for
the past couple of weeks :) Thanks a lot, Andrew!

This should be added to other Linux stable versions too, right?
(with the caveat that before 6.2, use class's lock instead of
pool's lock).

Reviewed-by: Nhat Pham <nphamcs@gmail.com>