Re: [PATCH v1 19/29] mm: stop storing migration_ops in page->mapping

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Jun 30, 2025 at 03:00:00PM +0200, David Hildenbrand wrote:
> ... instead, look them up statically based on the page type. Maybe in the
> future we want a registration interface? At least for now, it can be
> easily handled using the two page types that actually support page
> migration.
>
> The remaining usage of page->mapping is to flag such pages as actually
> being movable (having movable_ops), which we will change next.
>
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

See comment below, this feels iffy in the long run but ok as an interim measure.

So:

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

> ---
>  include/linux/balloon_compaction.h |  2 +-
>  include/linux/migrate.h            | 14 ++------------
>  include/linux/zsmalloc.h           |  2 ++
>  mm/balloon_compaction.c            |  1 -
>  mm/compaction.c                    |  5 ++---
>  mm/migrate.c                       | 23 +++++++++++++++++++++++
>  mm/zpdesc.h                        |  5 ++---
>  mm/zsmalloc.c                      |  8 +++-----
>  8 files changed, 35 insertions(+), 25 deletions(-)
>
> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> index 9bce8e9f5018c..a8a1706cc56f3 100644
> --- a/include/linux/balloon_compaction.h
> +++ b/include/linux/balloon_compaction.h
> @@ -92,7 +92,7 @@ static inline void balloon_page_insert(struct balloon_dev_info *balloon,
>  				       struct page *page)
>  {
>  	__SetPageOffline(page);
> -	__SetPageMovable(page, &balloon_mops);
> +	__SetPageMovable(page);
>  	set_page_private(page, (unsigned long)balloon);
>  	list_add(&page->lru, &balloon->pages);
>  }
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index e04035f70e36f..6aece3f3c8be8 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -104,23 +104,13 @@ static inline int migrate_huge_page_move_mapping(struct address_space *mapping,
>  #endif /* CONFIG_MIGRATION */
>
>  #ifdef CONFIG_COMPACTION
> -void __SetPageMovable(struct page *page, const struct movable_operations *ops);
> +void __SetPageMovable(struct page *page);
>  #else
> -static inline void __SetPageMovable(struct page *page,
> -		const struct movable_operations *ops)
> +static inline void __SetPageMovable(struct page *page)
>  {
>  }
>  #endif
>
> -static inline
> -const struct movable_operations *page_movable_ops(struct page *page)
> -{
> -	VM_WARN_ON_ONCE_PAGE(!page_has_movable_ops(page), page);
> -
> -	return (const struct movable_operations *)
> -		((unsigned long)page->mapping - PAGE_MAPPING_MOVABLE);
> -}
> -
>  #ifdef CONFIG_NUMA_BALANCING
>  int migrate_misplaced_folio_prepare(struct folio *folio,
>  		struct vm_area_struct *vma, int node);
> diff --git a/include/linux/zsmalloc.h b/include/linux/zsmalloc.h
> index 13e9cc5490f71..f3ccff2d966cd 100644
> --- a/include/linux/zsmalloc.h
> +++ b/include/linux/zsmalloc.h
> @@ -46,4 +46,6 @@ void zs_obj_read_end(struct zs_pool *pool, unsigned long handle,
>  void zs_obj_write(struct zs_pool *pool, unsigned long handle,
>  		  void *handle_mem, size_t mem_len);
>
> +extern const struct movable_operations zsmalloc_mops;
> +
>  #endif
> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> index e4f1a122d786b..2a4a649805c11 100644
> --- a/mm/balloon_compaction.c
> +++ b/mm/balloon_compaction.c
> @@ -253,6 +253,5 @@ const struct movable_operations balloon_mops = {
>  	.isolate_page = balloon_page_isolate,
>  	.putback_page = balloon_page_putback,
>  };
> -EXPORT_SYMBOL_GPL(balloon_mops);
>
>  #endif /* CONFIG_BALLOON_COMPACTION */
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 41fd6a1fe9a33..348eb754cb227 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -114,11 +114,10 @@ static unsigned long release_free_list(struct list_head *freepages)
>  }
>
>  #ifdef CONFIG_COMPACTION
> -void __SetPageMovable(struct page *page, const struct movable_operations *mops)
> +void __SetPageMovable(struct page *page)
>  {
>  	VM_BUG_ON_PAGE(!PageLocked(page), page);
> -	VM_BUG_ON_PAGE((unsigned long)mops & PAGE_MAPPING_MOVABLE, page);
> -	page->mapping = (void *)((unsigned long)mops | PAGE_MAPPING_MOVABLE);
> +	page->mapping = (void *)(PAGE_MAPPING_MOVABLE);
>  }
>  EXPORT_SYMBOL(__SetPageMovable);
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 15d3c1031530c..c6c9998014ec8 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -43,6 +43,8 @@
>  #include <linux/sched/sysctl.h>
>  #include <linux/memory-tiers.h>
>  #include <linux/pagewalk.h>
> +#include <linux/balloon_compaction.h>
> +#include <linux/zsmalloc.h>
>
>  #include <asm/tlbflush.h>
>
> @@ -51,6 +53,27 @@
>  #include "internal.h"
>  #include "swap.h"
>
> +static const struct movable_operations *page_movable_ops(struct page *page)
> +{
> +	VM_WARN_ON_ONCE_PAGE(!page_has_movable_ops(page), page);
> +
> +	/*
> +	 * If we enable page migration for a page of a certain type by marking
> +	 * it as movable, the page type must be sticky until the page gets freed
> +	 * back to the buddy.
> +	 */

Ah now this makes more sense...

> +#ifdef CONFIG_BALLOON_COMPACTION
> +	if (PageOffline(page))
> +		/* Only balloon compaction sets PageOffline pages movable. */
> +		return &balloon_mops;

So it's certain that if we try to invoke movable ops, and it's the balloon
compaction case, the page will be offline?

> +#endif /* CONFIG_BALLOON_COMPACTION */
> +#if defined(CONFIG_ZSMALLOC) && defined(CONFIG_COMPACTION)
> +	if (PageZsmalloc(page))

And same question only for ZS malloc.

> +		return &zsmalloc_mops;
> +#endif /* defined(CONFIG_ZSMALLOC) && defined(CONFIG_COMPACTION) */
> +	return NULL;
> +}

This is kind of sketchy as it's baking in assumptions implicitly, so I hope we
can find an improved way of doing this later, even if it's about providing
e.g. is_ballon_movable_ops_page() and is_zsmalloc_movable_ops_page() predicates
that abstract this code + placing them in the relevant code so it's at least
obvious to people working on this stuff that this needs to be considered.

But ok as a means of getting away from having to have the hook object encoded.

> +
>  /**
>   * isolate_movable_ops_page - isolate a movable_ops page for migration
>   * @page: The page.
> diff --git a/mm/zpdesc.h b/mm/zpdesc.h
> index 5763f36039736..6855d9e2732d8 100644
> --- a/mm/zpdesc.h
> +++ b/mm/zpdesc.h
> @@ -152,10 +152,9 @@ static inline struct zpdesc *pfn_zpdesc(unsigned long pfn)
>  	return page_zpdesc(pfn_to_page(pfn));
>  }
>
> -static inline void __zpdesc_set_movable(struct zpdesc *zpdesc,
> -					const struct movable_operations *mops)
> +static inline void __zpdesc_set_movable(struct zpdesc *zpdesc)
>  {
> -	__SetPageMovable(zpdesc_page(zpdesc), mops);
> +	__SetPageMovable(zpdesc_page(zpdesc));
>  }
>
>  static inline void __zpdesc_set_zsmalloc(struct zpdesc *zpdesc)
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 72c2b7562c511..7192196b9421d 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1684,8 +1684,6 @@ static void lock_zspage(struct zspage *zspage)
>
>  #ifdef CONFIG_COMPACTION
>
> -static const struct movable_operations zsmalloc_mops;
> -
>  static void replace_sub_page(struct size_class *class, struct zspage *zspage,
>  				struct zpdesc *newzpdesc, struct zpdesc *oldzpdesc)
>  {
> @@ -1708,7 +1706,7 @@ static void replace_sub_page(struct size_class *class, struct zspage *zspage,
>  	set_first_obj_offset(newzpdesc, first_obj_offset);
>  	if (unlikely(ZsHugePage(zspage)))
>  		newzpdesc->handle = oldzpdesc->handle;
> -	__zpdesc_set_movable(newzpdesc, &zsmalloc_mops);
> +	__zpdesc_set_movable(newzpdesc);
>  }
>
>  static bool zs_page_isolate(struct page *page, isolate_mode_t mode)
> @@ -1815,7 +1813,7 @@ static void zs_page_putback(struct page *page)
>  {
>  }
>
> -static const struct movable_operations zsmalloc_mops = {
> +const struct movable_operations zsmalloc_mops = {
>  	.isolate_page = zs_page_isolate,
>  	.migrate_page = zs_page_migrate,
>  	.putback_page = zs_page_putback,
> @@ -1878,7 +1876,7 @@ static void SetZsPageMovable(struct zs_pool *pool, struct zspage *zspage)
>
>  	do {
>  		WARN_ON(!zpdesc_trylock(zpdesc));
> -		__zpdesc_set_movable(zpdesc, &zsmalloc_mops);
> +		__zpdesc_set_movable(zpdesc);
>  		zpdesc_unlock(zpdesc);
>  	} while ((zpdesc = get_next_zpdesc(zpdesc)) != NULL);
>  }
> --
> 2.49.0
>