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

Lorenzo Stoakes
2025-07-01T12:46:53Z

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@nvidia.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Seems reasonable though, so:

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

> ---
> 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
>