Re: [PATCH v1 12/29] mm/zsmalloc: stop using __ClearPageMovable()

[Lorenzo Stoakes]

On Mon, Jun 30, 2025 at 02:59:53PM +0200, David Hildenbrand wrote:
> Instead, let's check in the callbacks if the page was already destroyed,
> which can be checked by looking at zpdesc->zspage (see reset_zpdesc()).
>
> If we detect that the page was destroyed:
>
> (1) Fail isolation, just like the migration core would
>
> (2) Fake migration success just like the migration core would
>
> In the putback case there is nothing to do, as we don't do anything just
> like the migration core would do.
>
> In the future, we should look into not letting these pages get destroyed
> while they are isolated -- and instead delaying that to the
> putback/migration call. Add a TODO for that.
>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

> ---
>  mm/zsmalloc.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index f98747aed4330..72c2b7562c511 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -876,7 +876,6 @@ static void reset_zpdesc(struct zpdesc *zpdesc)
>  {
>  	struct page *page = zpdesc_page(zpdesc);
>
> -	__ClearPageMovable(page);
>  	ClearPagePrivate(page);
>  	zpdesc->zspage = NULL;
>  	zpdesc->next = NULL;
> @@ -1715,10 +1714,11 @@ static void replace_sub_page(struct size_class *class, struct zspage *zspage,
>  static bool zs_page_isolate(struct page *page, isolate_mode_t mode)
>  {
>  	/*
> -	 * Page is locked so zspage couldn't be destroyed. For detail, look at
> -	 * lock_zspage in free_zspage.
> +	 * Page is locked so zspage can't be destroyed concurrently
> +	 * (see free_zspage()). But if the page was already destroyed
> +	 * (see reset_zpdesc()), refuse isolation here.
>  	 */
> -	return true;
> +	return page_zpdesc(page)->zspage;
>  }
>
>  static int zs_page_migrate(struct page *newpage, struct page *page,
> @@ -1736,6 +1736,13 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
>  	unsigned long old_obj, new_obj;
>  	unsigned int obj_idx;
>
> +	/*
> +	 * TODO: nothing prevents a zspage from getting destroyed while
> +	 * isolated: we should disallow that and defer it.
> +	 */
> +	if (!zpdesc->zspage)
> +		return MIGRATEPAGE_SUCCESS;
> +
>  	/* The page is locked, so this pointer must remain valid */
>  	zspage = get_zspage(zpdesc);
>  	pool = zspage->pool;
> --
> 2.49.0
>