Re: [PATCH v1 20/29] mm: convert "movable" flag in page->mapping to a page flag

[Lorenzo Stoakes]

On Mon, Jun 30, 2025 at 03:00:01PM +0200, David Hildenbrand wrote:
> Instead, let's use a page flag. As the page flag can result in
> false-positives, glue it to the page types for which we
> support/implement movable_ops page migration.
>
> The flag reused by PageMovableOps() might be sued by other pages, so

I assume 'used' not 'sued' :P

> warning in case it is set in page_has_movable_ops() might result in
> false-positive warnings.

Worth mentioning that it's PG_uptodate. Also probably worth putting a proviso
here that we're safe to use it for movable ops pages because it's used to track
file system state.

>
> Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Seems reasonable though, so:

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

> ---
>  include/linux/balloon_compaction.h |  2 +-
>  include/linux/migrate.h            |  8 -----
>  include/linux/page-flags.h         | 52 ++++++++++++++++++++++++------
>  mm/compaction.c                    |  6 ----
>  mm/zpdesc.h                        |  2 +-
>  5 files changed, 44 insertions(+), 26 deletions(-)
>
> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> index a8a1706cc56f3..b222b0737c466 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);
> +	SetPageMovableOps(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 6aece3f3c8be8..acadd41e0b5cf 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -103,14 +103,6 @@ static inline int migrate_huge_page_move_mapping(struct address_space *mapping,
>
>  #endif /* CONFIG_MIGRATION */
>
> -#ifdef CONFIG_COMPACTION
> -void __SetPageMovable(struct page *page);
> -#else
> -static inline void __SetPageMovable(struct page *page)
> -{
> -}
> -#endif
> -
>  #ifdef CONFIG_NUMA_BALANCING
>  int migrate_misplaced_folio_prepare(struct folio *folio,
>  		struct vm_area_struct *vma, int node);
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 4c27ebb689e3c..016a6e6fa428a 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -170,6 +170,11 @@ enum pageflags {
>  	/* non-lru isolated movable page */
>  	PG_isolated = PG_reclaim,
>
> +#ifdef CONFIG_MIGRATION
> +	/* this is a movable_ops page (for selected typed pages only) */
> +	PG_movable_ops = PG_uptodate,
> +#endif
> +
>  	/* Only valid for buddy pages. Used to track pages that are reported */
>  	PG_reported = PG_uptodate,
>
> @@ -698,9 +703,6 @@ PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted)
>   * bit; and then folio->mapping points, not to an anon_vma, but to a private
>   * structure which KSM associates with that merged page.  See ksm.h.
>   *
> - * PAGE_MAPPING_KSM without PAGE_MAPPING_ANON is used for non-lru movable
> - * page and then folio->mapping points to a struct movable_operations.
> - *
>   * Please note that, confusingly, "folio_mapping" refers to the inode
>   * address_space which maps the folio from disk; whereas "folio_mapped"
>   * refers to user virtual address space into which the folio is mapped.
> @@ -743,13 +745,6 @@ static __always_inline bool PageAnon(const struct page *page)
>  {
>  	return folio_test_anon(page_folio(page));
>  }
> -
> -static __always_inline bool page_has_movable_ops(const struct page *page)
> -{
> -	return ((unsigned long)page->mapping & PAGE_MAPPING_FLAGS) ==
> -				PAGE_MAPPING_MOVABLE;
> -}
> -
>  #ifdef CONFIG_KSM
>  /*
>   * A KSM page is one of those write-protected "shared pages" or "merged pages"
> @@ -1133,6 +1128,43 @@ bool is_free_buddy_page(const struct page *page);
>
>  PAGEFLAG(Isolated, isolated, PF_ANY);
>
> +#ifdef CONFIG_MIGRATION
> +/*
> + * This page is migratable through movable_ops (for selected typed pages
> + * only).
> + *
> + * Page migration of such pages might fail, for example, if the page is
> + * already isolated by somebody else, or if the page is about to get freed.
> + *
> + * While a subsystem might set selected typed pages that support page migration
> + * as being movable through movable_ops, it must never clear this flag.
> + *
> + * This flag is only cleared when the page is freed back to the buddy.
> + *
> + * Only selected page types support this flag (see page_movable_ops()) and
> + * the flag might be used in other context for other pages. Always use
> + * page_has_movable_ops() instead.
> + */
> +PAGEFLAG(MovableOps, movable_ops, PF_NO_TAIL);
> +#else
> +PAGEFLAG_FALSE(MovableOps, movable_ops);
> +#endif
> +
> +/**
> + * page_has_movable_ops - test for a movable_ops page
> + * @page The page to test.
> + *
> + * Test whether this is a movable_ops page. Such pages will stay that
> + * way until freed.
> + *
> + * Returns true if this is a movable_ops page, otherwise false.
> + */
> +static inline bool page_has_movable_ops(const struct page *page)
> +{
> +	return PageMovableOps(page) &&
> +	       (PageOffline(page) || PageZsmalloc(page));
> +}
> +
>  static __always_inline int PageAnonExclusive(const struct page *page)
>  {
>  	VM_BUG_ON_PGFLAGS(!PageAnon(page), page);
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 348eb754cb227..349f4ea0ec3e5 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -114,12 +114,6 @@ static unsigned long release_free_list(struct list_head *freepages)
>  }
>
>  #ifdef CONFIG_COMPACTION
> -void __SetPageMovable(struct page *page)
> -{
> -	VM_BUG_ON_PAGE(!PageLocked(page), page);
> -	page->mapping = (void *)(PAGE_MAPPING_MOVABLE);
> -}
> -EXPORT_SYMBOL(__SetPageMovable);
>
>  /* Do not skip compaction more than 64 times */
>  #define COMPACT_MAX_DEFER_SHIFT 6
> diff --git a/mm/zpdesc.h b/mm/zpdesc.h
> index 6855d9e2732d8..25bf5ea0beb83 100644
> --- a/mm/zpdesc.h
> +++ b/mm/zpdesc.h
> @@ -154,7 +154,7 @@ static inline struct zpdesc *pfn_zpdesc(unsigned long pfn)
>
>  static inline void __zpdesc_set_movable(struct zpdesc *zpdesc)
>  {
> -	__SetPageMovable(zpdesc_page(zpdesc));
> +	SetPageMovableOps(zpdesc_page(zpdesc));
>  }
>
>  static inline void __zpdesc_set_zsmalloc(struct zpdesc *zpdesc)
> --
> 2.49.0
>